4-Channel, 200 kSPS 12-Bit ADC with Sequencer in 16-Lead TSSOP AD7923 FEATURES Fast throughput rate: 200 kSPS Specified for AVDD of 2.7 V to 5.25 V Low power 3.6 mW max at 200 kSPS with 3 V supply 7.5 mW max at 200 kSPS with 5 V supply 4 (single-ended) inputs with sequencer Wide input bandwidth 70 dB Min SNR at 50 kHz input frequency Flexible power/serial clock speed management No pipeline delays FUNCTIONAL BLOCK DIAGRAM AVDD REFIN VIN0 • • • • • • • • • • • • • VIN3 High speed serial interface SPI®-/QSPITM-/ MICROWIRETM-/DSP-compatible Shutdown mode: 0.5 μA max 16-lead TSSOP package Qualified for automotive applications T/H 12-BIT SUCCESSIVE APPROXIMATION ADC I/P MUX SCLK DOUT CONTROL LOGIC SEQUENCER DIN CS GENERAL DESCRIPTION The AD7923 is a 12-bit, high speed, low power, 4-channel, successive approximation (SAR) ADC. It operates from a single 2.7 V to 5.25 V power supply and features throughput rates up to 200 kSPS. It contains a low noise, wide bandwidth track-and-hold amplifier that can handle input frequencies in excess of 8 MHz. The conversion process and data acquisition are controlled by CS and the serial clock, allowing the device to easily interface with microprocessors or DSPs. The input signal is sampled on the falling edge of CS; the conversion is also initiated at this point. The AD7923 uses advanced design techniques to achieve very low power dissipation at maximum throughput rates. At maximum throughput rates, it consumes 1.2 mA maximum with 3 V supplies and 1.5 mA maximum with 5 V supplies. Through the configuration of the control register, the analog input range can be selected as 0 V to REFIN or 0 V to 2 × REFIN, with either straight binary or twos complement output coding. The AD7923 features four single-ended analog inputs with a channel sequencer to allow a preprogrammed selection of channels to be converted sequentially. The conversion time for the AD7923 is determined by the serial clock, SCLK, frequency, since this is used as the master clock to control the conversion. The conversion time can be as short as 800 ns with a 20 MHz SCLK. VDRIVE GND 03086-001 AD7923 Figure 1. PRODUCT HIGHLIGHTS 1. High Throughput with Low Power Consumption. The AD7923 offers up to 200 kSPS throughput rates. At the maximum throughput rate with 3 V supplies, the AD7923 dissipates just 3.6 mW of power. 2. Four Single-Ended Inputs with a Channel Sequencer. 3. Single-Supply Operation with VDRIVE Function. The VDRIVE function allows the serial interface to connect directly to either 3 V or 5 V processor systems independent of AVDD. 4. Flexible Power/Serial Clock Speed Management. The conversion rate is determined by the serial clock, allowing the conversion time to be reduced through the serial clock speed increase. The part also features various shutdown modes to maximize power efficiency at lower throughput rates. Current consumption is 0.5 μA maximum when in full shutdown. 5. No Pipeline Delay. The part features a SAR ADC with accurate control of the sampling instant via a CS input and once off conversion control. Rev. C Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2002–2011 Analog Devices, Inc. All rights reserved. AD7923 TABLE OF CONTENTS Specifications..................................................................................... 3 Typical Connection Diagram ................................................... 16 Timing Specifications....................................................................... 5 Modes of Operation ................................................................... 17 Absolute Maximum Ratings............................................................ 6 Powering Up the AD7923 ......................................................... 18 ESD Caution.................................................................................. 6 Power vs. Throughput Rate....................................................... 19 Pin Configuration and Function Description .............................. 7 Serial Interface ............................................................................ 20 Typical Performance Characteristics ............................................. 8 Microprocessor Interfacing....................................................... 21 Terminology .................................................................................... 10 Application Hints ........................................................................... 23 Control Register Descriptions ...................................................... 12 Grounding and Layout .............................................................. 23 Sequencer Operation ................................................................. 13 Evaluating the AD7923 Performance ...................................... 23 Theory of Operation ...................................................................... 14 Outline Dimensions ....................................................................... 24 Circuit Information.................................................................... 14 Ordering Guide .......................................................................... 24 Converter Operation.................................................................. 14 Automotive Products ................................................................. 24 ADC Transfer Function............................................................. 15 REVISION HISTORY 5/11—Rev. B to Rev. C Changes to Features Section............................................................ 1 Updated Outline Dimensions ....................................................... 24 Changes to Ordering Guide .......................................................... 24 Added to Automotive Products Section...................................... 24 12/08—Rev. A to Rev. B Changes to ESD Parameter, Table 3 ............................................... 6 Changes to Ordering Guide .......................................................... 24 8/05—Rev. 0 to Rev. A Update Format ....................................................................Universal Change to Table 1 ............................................................................. 3 Change to Table 3 ............................................................................. 6 Change to Reference Section ........................................................ 16 Changes to Ordering Guide .......................................................... 24 11/02—Revision 0: Initial Version Rev. C | Page 2 of 24 AD7923 SPECIFICATIONS AVDD = VDRIVE = 2.7 V to 5.25 V, REFIN = 2.5 V, fSCLK = 20 MHz, TA = TMIN to TMAX, unless otherwise noted. Table 1. Parameter DYNAMIC PERFORMANCE Signal-to-(Noise + Distortion) (SINAD) 2 Signal-to-Noise (SNR)2 Total Harmonic Distortion (THD)2 Peak Harmonic or Spurious Noise (SFDR)2 Intermodulation Distortion (IMD)2 Second Order Terms Third Order Terms Aperture Delay Aperture Jitter Channel-to-Channel Isolation Full Power Bandwidth DC ACCURACY2 Resolution Integral Nonlinearity Differential Nonlinearity 0 V to REFIN Input Range Offset Error Offset Error Match Gain Error Gain Error Match 0 V to 2 × REFIN Input Range Positive Gain Error Positive Gain Error Match Zero-Code Error Zero-Code Error Match Negative Gain Error Negative Gain Error Match ANALOG INPUT Input Voltage Range DC Leakage Current Input Capacitance REFERENCE INPUT REFIN Input Voltage DC Leakage Current REFIN Input Impedance LOGIC INPUTS Input High Voltage, VINH Input Low Voltage, VINL Input Current, IIN Input Capacitance, CIN 3 B Version 1 Unit 70 69 69 70 −77 −73 −78 −76 dB min dB min dB min dB min dB max dB max dB max dB max −90 −90 10 50 −85 8.2 1.6 dB typ dB typ ns typ ps typ dB typ MHz typ MHz typ 12 ±1 −0.9/+1.5 Bits LSB max LSB max ±8 ±0.5 ±1.5 ±0.5 LSB max LSB max LSB max LSB max Test Conditions/Comments fIN = 50 kHz sine wave, fSCLK = 20 MHz @ 5 V, –40°C to +85°C @ 5 V, 85°C to 125°C, typ 70 dB @ 3 V typ 70 dB, –40°C to +125°C @ 5 V typ, −84 dB @ 3 V typ,−77 dB @ 5 V typ, −86 dB @ 3 V typ, −80 dB fA = 40.1 kHz, fB = 41.5 kHz fIN = 400 kHz @ 3 dB @ 0.1 dB Guaranteed no missed codes to 12 bits Straight binary output coding Typ ±0.5 LSB −REFIN to +REFIN biased about REFIN with twos complement output coding ±1.5 ±0.5 ±8 ±0.5 ±1 ±0.5 LSB max LSB max LSB max LSB max LSB max LSB max 0 to REFIN 0 to 2 × REFIN ±1 20 V V μA max pF typ Range bit set to 1 Range bit set to 0, AVDD = 4.75 V to 5.25 V 2.5 ±1 36 V μA max kΩ typ ±1% specified performance 0.7 × VDRIVE 0.3 × VDRIVE ±1 10 V min V max μA max pF max Rev. C | Page 3 of 24 Typ ±0.8 LSB fSAMPLE = 200 kSPS Typ 10 nA, VIN = 0 V or VDRIVE AD7923 Parameter LOGIC OUTPUTS Output High Voltage, VOH Output Low Voltage, VOL Floating-State Leakage Current Floating-State Output Capacitance3 Output Coding CONVERSION RATE Conversion Time Track-and-Hold Acquisition Time Throughput Rate POWER REQUIREMENTS AVDD VDRIVE IDD 4 During Conversion Normal Mode (Static) Normal Mode (Operational) fSAMPLE = 200 kSPS Using Auto Shutdown Mode fSAMPLE = 200 kSPS Auto Shutdown (Static) Full Shutdown Mode Power Dissipation4 Normal Mode (Operational) fSAMPLE = 200 kSPS Auto Shutdown (Static) Full Shutdown Mode B Version 1 Unit Test Conditions/Comments VDRIVE – 0.2 0.4 ±1 10 Twos Complement Straight (Natural) Binary V min V max μA max pF max ISOURCE = 200 μA, AVDD = 2.7 V to 5.25 V ISINK = 200 μA 800 300 300 200 ns max ns max ns max kSPS max 2.7/5.25 2.7/5.25 V min/max V min/max 2.7 2.0 600 1.5 1.2 900 650 0.5 0.5 mA max mA max μA typ mA max mA max μA typ μA typ μA max μA max Digital I/Ps = 0 V or VDRIVE AVDD = 4.75 V to 5.25 V, fSCLK = 20 MHz AVDD = 2.7 V to 3.6 V, fSCLK = 20 MHz AVDD = 2.7 V to 5.25 V, SCLK on or off AVDD = 4.75 V to 5.25 V, fSCLK = 20 MHz AVDD = 2.7 V to 3.6 V, fSCLK = 20 MHz AVDD = 4.75 V to 5.25 V, fSAMPLE = 200 kSPS AVDD = 2.7 V to 3.6 V, fSAMPLE = 200 kSPS SCLK on or off (20 nA typ) SCLK on or off (20 nA typ) 7.5 3.6 2.5 1.5 2.5 1.5 mW max mW max μW max μW max μW max μW max AVDD = 5 V, fSCLK = 20 MHz AVDD = 3 V, fSCLK = 20 MHz AVDD = 5 V AVDD = 3 V AVDD = 5 V AVDD = 3 V Coding bit set to 0 Coding bit set to 1 1 Temperature range: B Version: −40°C to +125°C. See Terminology section. 3 Sample tested @ 25°C to ensure compliance. 4 See Power vs. Throughput Rate section. 2 Rev. C | Page 4 of 24 16 SCLK cycles with SCLK at 20 MHz Sinewave input Full-scale step Input See Serial Interface section AD7923 TIMING SPECIFICATIONS AVDD = 2.7 V to 5.25 V, VDRIVE ≤ AVDD, REFIN = 2.5 V, TA = TMIN to TMAX, unless otherwise noted. 1 Table 2. Parameter fSCLK 2 tCONVERT tQUIET AVDD = 3 V 10 20 16 × tSCLK 50 t2 t3 3 t43 t5 t6 t7 t8 4 t9 t10 t11 t12 10 35 40 0.4 × tSCLK 0.4 × tSCLK 10 15/45 10 5 20 1 Limit at TMIN, TMAX AVDD = 5 V Unit 10 kHz min 20 MHz max 16 × tSCLK 50 ns min 10 30 40 0.4 × tSCLK 0.4 × tSCLK 10 15/35 10 5 20 1 ns min ns max ns max ns min ns min ns min ns min/max ns min ns min ns min μs max Description Minimum quiet time required between CS rising edge and start of next conversion CS to SCLK set-up time Delay from CS until DOUT three-state disabled Data access time after SCLK falling edge SCLK low pulse width SCLK high pulse width SCLK to DOUT valid hold time SCLK falling edge to DOUT high impedance DIN set-up time prior to SCLK falling edge DIN hold time after SCLK falling edge Sixteenth SCLK falling edge to CS high Power-Up time from full power-down/auto shutdown mode 1 Sample tested at 25°C to ensure compliance. All input signals are specified with tR = tF = 5 ns (10% to 90% of AVDD) and timed from a voltage level of 1.6 V. See Figure 2. The 3 V operating range spans from 2.7 V to 3.6 V. The 5 V operating range spans from 4.75 V to 5.25 V. 2 The mark/space ratio for the SCLK input is 40/60 to 60/40. 3 Measured with the load circuit of Figure 2 and defined as the time required for the output to cross 0.4 V or 0.7 × VDRIVE. 4 t8 is derived from the measured time taken by the data outputs to change 0.5 V when loaded with the circuit of Figure 2. The measured number is then extrapolated back to remove the effects of charging or discharging the 50 pF capacitor. This means that the time, quoted in the timing characteristics t8, is the true bus relinquish time of the part and is independent of the bus loading. 200μA 1.6V CL 50pF 200μA IOH 03086-002 TO OUTPUT PIN IOL Figure 2. Load Circuit for Digital Output Timing Specification Rev. C | Page 5 of 24 AD7923 ABSOLUTE MAXIMUM RATINGS TA = 25°C, unless otherwise noted. Table 3. Parameter AVDD to AGND VDRIVE to AGND Analog Input Voltage to AGND Digital Input Voltage to AGND Digital Output Voltage to AGND REFIN to AGND Input Current to Any Pin Except Supplies 1 Operating Temperature Range Commercial (B Version) Storage Temperature Range Junction Temperature TSSOP Package, Power Dissipation θJA Thermal Impedance θJC Thermal Impedance Lead Temperature, Soldering Vapor Phase (60 sec) Infrared (15 sec) Pb-free Temperature, Soldering Reflow ESD 1 Rating −0.3 V to +7 V −0.3 V to AVDD + 0.3 V −0.3 V to AVDD + 0.3 V −0.3 V to +7 V −0.3 V to AVDD + 0.3 V −0.3 V to AVDD + 0.3 V ±10 mA Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. −40°C to +125°C −65°C to +150°C 150°C 450 mW 150.4°C/W (TSSOP) 27.6°C/W (TSSOP) 215°C 220°C 260(+0)°C 1.5 kV Transient currents of up to 100 mA do not cause SCR latchup. ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. Rev. C | Page 6 of 24 AD7923 PIN CONFIGURATION AND FUNCTION DESCRIPTION SCLK 1 16 AGND DIN 2 15 VDRIVE CS 3 AD7923 AGND 4 TOP VIEW (Not to Scale) AVDD 5 12 VIN0 AVDD 6 11 VIN1 REFIN 7 10 VIN2 AGND 8 14 DOUT 9 VIN3 03086-003 13 AGND Figure 3. Pin Configuration Table 4. Pin Function Descriptions Pin No. 1 Mnemonic SCLK 2 DIN 3 CS 4, 8, 13, 16 5, 6 AGND 7 REFIN 12 to 9 VIN0 to VIN3 14 DOUT 15 VDRIVE AVDD Function Serial Clock. Logic Input. SCLK provides the serial clock for accessing data for the part. This clock input is also used as the clock source for the AD7923 conversion process. Data In. Logic Input. Data to be written to the control register is provided on this input and is clocked into the register on the falling edge of SCLK (see the Control Register Descriptions section). Chip Select. Active low logic input. This input provides the dual function of initiating conversions on the AD7923 and framing the serial data transfer. Analog Ground. Ground reference point for all analog circuitry on the AD7923. All analog input signals and any external reference signal should be referred to this AGND voltage. All AGND pins should be connected together. Analog Power Supply Input. The AVDD range for the AD7923 is from 2.7 V to 5.25 V. For the 0 V to 2 × REFIN range, AVDD should be from 4.75 V to 5.25 V. Reference Input for the AD7923. An external reference must be applied to this input. The voltage range for the external reference is 2.5 V ± 1% for specified performance. Analog Input 0 through Analog Input 3. Four single-ended analog input channels that are multiplexed into the onchip track-and-hold. The analog input channel to be converted is selected by using the Address Bits ADD1 and ADD0 of the control register. The address bits in conjunction with the SEQ1 and SEQ0 bits allow the sequencer to be programmed. The input range for all input channels can extend from 0 V to REFIN or from 0 V to 2 × REFIN as selected via the range bit in the control register. Any unused input channels must be connected to AGND to avoid noise pickup. Data Out. Logic Output. The conversion result from the AD7923 is provided on this output pin as a serial data stream. The AD7923 serial data stream consists of two leading 0s, and two address bits indicating which channel the conversion result corresponds to, followed by 12 bits of conversion data, MSB first. The output coding can be selected as straight binary or twos complement via the coding bit in the control register. The data bits are clocked out of the AD7923 on the SCLK falling edge. Logic Power Supply Input. The voltage supplied at this pin determines at which voltage the serial interface operates. Rev. C | Page 7 of 24 AD7923 TYPICAL PERFORMANCE CHARACTERISTICS –50 4096 POINT FFT AVDD = 4.75V fSAMPLE = 200kSPS fIN = 50 kHz SINAD = 70.714dB THD = –82.853dB SFDR = –84.815dB –10 –60 –65 THD (dB) SNR (dB) –30 fSAMPLE = 200kSPS TA = 25°C RANGE = 0V TO REFIN –55 –50 –70 AVDD = VDRIVE = 2.7V –70 AVDD = VDRIVE = 3.6V –75 –80 03096-004 –85 –110 0 10 20 30 40 50 60 70 FREQUENCY (kHz) 80 90 AVDD = VDRIVE = 4.75V AVDD = VDRIVE = 5.25V –90 10 100 03086-007 –90 100 INPUT FREQUENCY (kHz) Figure 4. Dynamic Performance at 200 kSPS Figure 7. THD vs. Analog Input Frequency for Various Supply Voltages at 200 kSPS 75 –55 fSAMPLE = 200kSPS TA = 25°C AVDD = 5.25V RANGE = 0V TO REFIN –60 AVDD = VDRIVE = 5.25V AVDD = VDRIVE = 4.75V –65 70 THD (dB) SINAD (dB) –70 AVDD = VDRIVE = 3.6V AVDD = VDRIVE = 2.7V RIN = 1000Ω –75 –80 RIN = 100Ω 65 RIN = 10Ω –85 60 0 RIN = 50Ω –95 10 100 100 INPUT FREQUENCY (kHz) INPUT FREQUENCY (kHz) Figure 5. SINAD vs. Analog Input Frequency for Various Supply Voltages at 200 kSPS Figure 8. THD vs. Analog Input Frequency for Various Source Impedances 0 1.0 AVDD = 5V 200mV p-p SINE WAVE ON AVDD REFIN = 2.5V, 1μF CAPACITOR TA = 25°C –10 AVDD = VDRIVE = 5V TEMP = 25°C 0.8 0.6 INL ERROR (LSB) –20 –30 –40 –50 –60 0.4 0.2 0 –0.2 –0.4 –70 –80 –90 0 20 40 60 80 100 120 140 160 SUPPLY RIPPLE FREQUENCY (kHz) 180 03096-009 –0.6 03086-006 PSRR (dB) 03086-008 –90 03086-005 fSAMPLE = 200kSPS TA = 25°C RANGE = 0V TO REFIN –0.8 –1.0 0 200 512 1024 1536 2048 CODE 2560 Figure 9. Typical INL Figure 6. PSRR vs. Supply Ripple Frequency Rev. C | Page 8 of 24 3072 3584 4096 AD7923 1.0 AVDD = VDRIVE = 5V TEMP = 25°C 0.8 0.4 0.2 0 –0.2 –0.4 –0.6 03096-010 DNL ERROR (LSB) 0.6 –0.8 –1.0 0 512 1024 1536 2048 CODE 2560 3072 3584 4096 Figure 10. Typical DNL Rev. C | Page 9 of 24 AD7923 TERMINOLOGY Integral Nonlinearity This is the maximum deviation from a straight line passing through the endpoints of the ADC transfer function. The endpoints of the transfer function are zero scale, a point 1 LSB below the first code transition, and full scale, a point 1 LSB above the last code transition. Differential Nonlinearity This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC. Offset Error This is the deviation of the first code transition (00 ... 000) to (00 ... 001) from the ideal, that is, AGND + 1 LSB. Offset Error Match This is the difference in offset error between any two channels. Gain Error This is the deviation of the last code transition (111 ... 110) to (111 ... 111) from the ideal (that is, REFIN – 1 LSB) after the offset error has been adjusted. Gain Error Match This is the difference in gain error between any two channels. Zero-Code Error This applies when using the twos complement output coding option, in particular, with the 2 × REFIN input range when −REFIN to +REFIN is biased around the REFIN point. It defined as the deviation of the midscale transition (all 0s to all 1s) from the ideal VIN voltage, that is, REFIN – 1 LSB. Zero-Code Error Match This is the difference in zero-code error between any two channels. Positive Gain Error This applies when using the twos complement output coding option, in particular, with the 2 × REFIN input range when −REFIN to +REFIN is biased around the REFIN point. It is the deviation of the last code transition (011 ... 110) to (011 ... 111) from the ideal (that is, +REFIN – 1 LSB) after the zero-code error has been adjusted. Positive Gain Error Match This is the difference in positive gain error between any two channels. Negative Gain Error This applies when using the twos complement output coding option, in particular, with the 2 × REFIN input range when −REFIN to +REFIN is biased around the REFIN point. It is the deviation of the first code transition (100 ... 000) to (100 ... 001) from the ideal (that is, −REFIN + 1 LSB) after the zero-code error has been adjusted. Negative Gain Error Match This is the difference in negative gain error between any two channels. Channel-to-Channel Isolation Channel-to-channel isolation is a measure of the level of crosstalk between channels. It is measured by applying a full-scale 400 kHz sine wave signal to all three nonselected input channels and determining how much that signal is attenuated in the selected channel with a 50 kHz signal. The figure is given in the worst-case across all four channels for the AD7923. Power Supply Rejection (PSR) Variations in power supply affect the full-scale transition, but not the converter’s linearity. Power supply rejection is the maximum change in the full-scale transition point from a change in power supply voltage from the nominal value. Figure 6 shows the power supply rejection ratio vs. supply ripple frequency for the AD7923 with no decoupling. The power supply rejection ratio is defined as the ratio of the power in the ADC output at full-scale frequency, f, to the power of a 200 mV p-p sine wave applied to the ADC AVDD supply of frequency fS: PSSR (dB) = 10log(Pf/PfS) Pf is equal to the power at frequency f in the ADC output; PfS is equal to the power at frequency fS coupled onto the ADC AVDD supply. Track-and-Hold Acquisition Time The track-and-hold amplifier returns into track mode at the end of conversion. Track-and-hold acquisition time is the time required for the output of the track-and-hold amplifier to reach its final value, within ±1 LSB, after the end of conversion. Rev. C | Page 10 of 24 AD7923 Signal-to-(Noise + Distortion) (SINAD) Ratio This is the measured ratio of SINAD at the output of the ADC. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process, the more levels, the smaller the quantization noise. The theoretical SINAD ratio for an ideal N-bit converter with a sine wave input is given by SINAD = (6.02N + 1.76) dB Total Harmonic Distortion (THD) THD is the ratio of the rms sum of harmonics to the fundamental. For the AD7923, it is defined as THD(dB) = 20 log V22 + V32 + V42 + V52 + V62 V1 where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. Thus for a 12-bit converter, this is 74 dB. Rev. C | Page 11 of 24 AD7923 CONTROL REGISTER DESCRIPTIONS The control register on the AD7923 is a 12-bit, write-only register. Data is loaded from the DIN pin of the AD7923 on the falling edge of SCLK. The data is transferred on the DIN line at the same time that the conversion result is read from the part. The data transferred on the DIN line corresponds to the AD7923 configuration for the next conversion. This requires 16 serial clocks for every data transfer. Only the information provided on the first 12 falling clock edges (after CS falling edge) is loaded to the control register. MSB denotes the first bit in the data stream. The bit functions are outlined in Table 5. Table 5. Control Register Bit Functions MSB WRITE SEQ1 DONTC DONTC ADD1 ADD0 PM1 PM0 SEQ0 DONTC RANGE LSB CODING Table 6. Bit 11 Name WRITE 10 SEQ1 7–6 ADD1 ADD0 5, 4 3 PM1 PM0 SEQ0 2, 9–8 1 DONTC RANGE 0 CODING Description The value written to this bit of the control register determines whether the following 11 bits are loaded to the control register. If this bit is a 1, the following 11 bits are written to the control register. If it is a 0, the remaining 11 bits are not loaded to the control register and it remains unchanged. The SEQ1 bit in the control register is used with the SEQ0 bit to control the use of the sequencer function (see Table 9). These two address bits are loaded at the end of the present conversion and select which analog input channel is converted in the next serial transfer, or they can also be used to select the final channel in a consecutive sequence, as described in Table 9. The selected input channel is decoded, as shown in Table 7. The next channel to be converted on is selected by the mux on the 14th SCLK falling edge. Channel address bits corresponding to the conversion result are also output on the DOUT serial data stream prior to the 12 bits of data (see the Serial Interface section). Power management bits. These two bits decode the mode of operation of the AD7923, as shown in Table 8. The SEQ0 bit in the control register is used with the SEQ1 bit to control the use of the sequencer function. (see Table 9). Don’t care. This bit selects the analog input range to be used on the AD7923. If it is set to 0, the analog input range extends from 0 V to 2 × REFIN. If it is set to 1, the analog input range extends from 0 V to REFIN (for the next conversion). For the 0 V to 2 × REFIN range, AVDD = 4.75 V to 5.25 V. This bit selects the type of output coding the AD7923 uses for the conversion result. If this bit is set to 0, the output coding for the part is twos complement. If this bit is set to 1, the output coding from the part is straight binary (for the next conversion). Table 7. Channel Selection ADD1 0 0 1 1 ADD0 0 1 0 1 Analog Input Channel VIN0 VIN1 VIN2 VIN3 Rev. C | Page 12 of 24 AD7923 Table 8. Power Mode Selection PM1 1 PM0 1 1 0 0 1 0 0 Mode Normal operation. In this mode, the AD7923 remains in full power mode, regardless of the status of any of the logic inputs. This mode allows the fastest possible throughput rate from the AD7923. Full shutdown. In this mode, the AD7923 is in full shutdown mode with all circuitry on the AD7923 powering down. The AD7923 retains the information in the control register while in full shutdown. The part remains in full shutdown until these bits are changed. Auto shutdown. In this mode, the AD7923 automatically enters full shutdown mode at the end of each conversion when the control register is updated. Wake-up time from full shutdown is 1 μs, and the user should ensure that 1 μs has elapsed before attempting to perform a valid conversion on the part in this mode. Invalid selection. This configuration is not allowed. SEQUENCER OPERATION The configuration of the SEQ1 and SEQ0 bits in the control register allows the user to select a particular mode of operation of the sequencer function. Table 9 outlines the three modes of operation of the sequencer. Table 9. Sequence Selection SEQ0 X 1 0 1 1 Sequence Type This configuration means that the sequence function is not used. The analog input channel selected for each individual conversion is determined by the contents of Channel Address Bits ADD1 and ADD0 in each prior write operation. This mode of operation reflects the traditional operation of a multichannel ADC without the sequencer function being used, where each write to the AD7923 selects the next channel for conversion (see Figure 11). If the SEQ1 and SEQ0 bits are set in this way, the sequence function is not interrupted upon completion of the write operation. This allows other bits in the control register to be altered between conversions while in a sequence without terminating the cycle. This configuration is used in conjunction with Channel Address Bits ADD1 and ADD0 to program continuous conversions on a consecutive sequence of channels from Channel 0 to a selected final channel as determined by the channel address bits in the control register (see Figure 12). Figure 11 reflects the traditional operation of a multichannel ADC, where each serial transfer selects the next channel for conversion. In this mode of operation the sequencer function is not used. Figure 12 shows how to program the AD7923 to continuously convert on a sequence of consecutive channels from Channel 0 to a selected final channel. To exit this mode of operation and revert back to the traditional mode of operation of a multichannel ADC (as outlined in Figure 11), ensure that the write bit = 1 and SEQ1 = SEQ0 = 0 on the next serial transfer. POWER-ON DUMMY CONVERSION CS DOUT: CONVERSION RESULT FROM CHANNEL 0 POWER-ON CS DUMMY CONVERSION CS DIN: WRITE TO CONTROL REGISTER, WRITE BIT = 1, SELECT CODING, RANGE, AND POWER MODE. SELECT CHANNEL A1, A0 FOR CONVERSION. SEQ1 = 0, SEQ0 = x CS DOUT: CONVERSION RESULT FROM PREVIOUSLY SELECTED CHANNEL A1, A0 DIN: WRITE TO CONTROL REGISTER, WRITE BIT = 1, SELECT CODING, RANGE, AND POWER MODE. SELECT CHANNEL A1, A0 FOR CONVERSION. SEQ1 = 0, SEQ0 = x CONTINUOUSLY CONVERTS ON THE SELECTED SEQUENCE OF CHANNELS FROM CHANNEL 0 UP TO AND INCLUDING THE PREVIOUSLY SELECTED A1, A0 IN THE CONTROL REGISGER CONTINUOUSLY CONVERTS ON THE SELECTED SEQUENCE OF CHANNELS BUT WILL ALLOW RANGE, CODING, ETC., TO CHANGE IN THE CONTROL REGISTER WITHOUT INTERRUPTING THE SEQUENCY, PROVIDED SEQ =1, SEQ0 = 0 WRITE BIT = 0 WRITE BIT = 1, SEQ1 = 1, SEQ0 = 0 Figure 12. SEQ1 Bit = 1, SEQ0 Bit = 1 Flowchart WRITE BIT = 1, SEQ1 = 0, SEQ0 = x 03086-011 CS DIN: WRITE TO CONTROL REGISTER, WRITE BIT = 1, SELECT CODING, RANGE, AND POWER MODE. SELECT CHANNEL A1, A0 FOR CONVERSION. SEQ1 = 1, SEQ0 = 1 Figure 11. SEQ1 Bit = 0, SEQ0 Bit = X Flowchart Rev. C | Page 13 of 24 03086-012 SEQ1 0 AD7923 THEORY OF OPERATION The AD7923 is a high speed, 4-channel, 12-bit single-supply ADC. The part can be operated from a 2.7 V to 5.25 V supply. When operated from either a 5 V or 3 V supply, the AD7923 is capable of throughput rates of 200 kSPS. The conversion time can be as short as 800 ns when provided with a 20 MHz clock. When the comparator is rebalanced, the conversion is complete. The control logic generates the ADC output code. Figure 16 and Figure 17 show the ADC transfer functions. CAPACITIVE DAC A VIN0 SW1 The AD7923 provides the user with an on-chip track-and-hold ADC and with a serial interface housed in a 16-lead TSSOP package. The AD7923 has four, single-ended input channels with a channel sequencer, allowing the user to select a channel sequence through which the ADC can cycle with each conseutive CS falling edge. The serial clock input accesses data from the part, controls the transfer of data written to the ADC, and provides the clock source for the successive approximation ADC. The analog input range is 0 V to REFIN or 0 V to 2 × REFIN, depending on the status of the RANGE bit in the control register. For the 0 to 2 × REFIN range, the part must be operated from a 4.75 V to 5.25 V AVDD supply. The AD7923 provides flexible power management options to allow the user to achieve the best power performance for a given throughput rate. These options are selected by programming the power management bits, PM1 and PM0, in the control register. CONVERTER OPERATION The AD7923 is a 12-bit successive approximation ADC based around a capacitive DAC. It can convert analog input signals in the range 0 V to REFIN or 0 V to 2 × REFIN. Figure 13 and Figure 14 show simplified schematics of the ADC. The ADC is comprised of a control logic, SAR, and capacitive DAC, which are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into a balanced condition. Figure 13 shows the ADC during its acquisition phase. SW2 is closed and SW1 is in Position A. The comparator is held in a balanced condition and the sampling capacitor acquires the signal on the selected VIN channel. CAPACITIVE DAC A SW1 VIN3 4kΩ B CONTROL LOGIC SW2 COMPARATOR AGND B CONTROL LOGIC SW2 VIN3 COMPARATOR AGND Figure 14. ADC Conversion Phase Analog Input Figure 15 shows an equivalent circuit of the analog input structure of the AD7923. The two diodes D1 and D2 provide ESD protection for the analog inputs. Care must be taken to ensure that the analog input signal never exceeds the supply rails by more than 200 mV; otherwise these diodes become forward-biased and start conducting current into the substrate. 10 mA is the maximum current these diodes can conduct without causing irreversible damage to the part. Capacitor C1, shown in Figure 15, is typically around 4 pF and can primarily be attributed to pin capacitance. The resistor R1 is a lumped component made up of the on resistance of the track-and-hold switch and includes the on resistance of the input multiplexer. The total resistance is typically about 400 Ω. Capacitor C2 is the ADC sampling capacitor and has a capacitance of 30 pF typically. For ac applications, removing high frequency components from the analog input signal is recommended by using an RC low-pass filter on the relevant analog input pin. In applications where harmonic distortion and the signal-to-noise ratio are critical, the analog input should be driven from a low impedance source. Large source impedances significantly affect the ac performance of the ADC. This may necessitate the use of an input buffer amplifier. The choice of the op amp is a function of the particular application. When no amplifier is used to drive the analog input, the source impedance should be limited to low values. The maximum source impedance depends on the amount of THD that can be tolerated. The THD increases as the source impedance increases and performance degrades (see Figure 8). AVDD 03086-013 VIN0 4kΩ 03086-014 CIRCUIT INFORMATION D1 R1 Figure 13. ADC Acquisition Phase C2 30pF VIN Rev. C | Page 14 of 24 C1 4pF D2 CONVERSION PHASE: SWITCH OPEN TRACK PHASE: SWITCH CLOSED 03086-015 When the ADC starts a conversion (see Figure 14), SW2 opens and SW1 moves to Position B, causing the comparator to become unbalanced. The control logic and the capacitive DAC are used to add and subtract fixed amounts of charge from the sampling capacitor to bring the comparator back into balance. Figure 15. Equivalent Analog Input Circuit AD7923 ADC TRANSFER FUNCTION 1LSB = 2 × VREF/4096 03086-017 ADC CODE 011...111 011...110 • • 000...001 000...000 111...111 • • 100...010 100...001 100...000 –VREF + 1LSB +VREF – 1LSB VREF – 1LSB ANALOG INPUT Figure 17. Twos Complement Transfer Characteristic with REFIN ± REFIN Input Range 111...111 111...110 • • 111...000 • 011...111 • • 000...010 000...001 000...000 Handling Bipolar Input Signals Figure 18 shows how useful the combination of the 2 × REFIN input range and the twos complement output coding scheme is for handling bipolar input signals. If the bipolar input signal is biased around REFIN and twos complement output coding is selected, then REFIN becomes the zero-code point, −REFIN is negative full scale, and + REFIN becomes positive full scale with a dynamic range of 2 × REFIN. 1LSB = VREF/4096 0V 1LSB +VREF – 1LSB ANALOG INPUT 03086-016 NOTES 1. VREF IS EITHER REFIN OR 2 × REFIN Figure 16. Straight Binary Transfer Characteristic VDD VREF 0.1μF REFIN AVDD VDD VDRIVE R4 AD7923 V R3 0V V VIN0 R2 R1 R1 = R2 = R3 = R4 DOUT • • VIN3 DSP/μP TWOS COMPLEMENT +REFIN (= 2 × REFIN) 000...000 REFIN –REFIN Figure 18. Handling Bipolar Signals Rev. C | Page 15 of 24 011...111 (= 0V) 100...000 03086-018 ADC CODE The output coding of the AD7923 is either straight binary or twos complement, depending on the status of the LSB in the control register. The designed code transitions occur at successive LSB values (for example, 1 LSB, 2 LSBs). The LSB size is REFIN /4096 for the AD7923. The ideal transfer characteristic for the AD7923 when straight binary coding is selected is shown in Figure 16 and the ideal transfer characteristic for the AD7923 when twos complement coding is selected is shown in Figure 17. AD7923 TYPICAL CONNECTION DIAGRAM Figure 19 shows a typical connection diagram for the AD7923. In this setup the AGND pin is connected to the analog ground plane of the system. In Figure 19, REFIN is connected to a decoupled 2.5 V supply from a reference source, the AD780, to provide an analog input range of 0 V to 2.5 V (if the range bit is 1) or 0 V to 5 V (if the range bit is 0). Although the AD7923 is connected to AVDD of 5 V, the serial interface is connected to a 3 V microprocessor. The VDRIVE pin of the AD7923 is connected to the same 3 V supply of the microprocessor to allow a 3 V logic interface (see the Digital Inputs section). The conversion result is output in a 16-bit word. This 16-bit data stream consists of two leading 0s, two address bits indicating which channel the conversion result corresponds to, followed by the 12 bits of conversion data. For applications where power consumption is a concern, the power-down modes should be used between conversions or bursts of several conversions to improve power performance. See the Modes of Operation section. 5V SUPPLY 0.1μF Digital Inputs The digital inputs applied to the AD7923 are not limited by the maximum ratings that limit the analog inputs. Instead, the digital inputs applied can go to 7 V and are not restricted by the AVDD + 0.3 V limit as on the analog inputs. AVDD AD7923 VIN3 AGND 0.1μF SCLK DOUT μC/μP CS VDRIVE DIN REFIN 2.5V AD780 0.1μF 10μF 3V SUPPLY 03086-019 0V TO REFIN Regardless of which channel selection method is used, the 16-bit word output from the AD7923 during each conversion always contains two leading 0s, and two channel address bits that the conversion result corresponds to, followed by the 12-bit conversion result. (see the Serial Interface section). 10μF SERIAL INTERFACE VIN0 • • the control register again once a sequencer operation has been initiated. The write bit must be set to 0 or the DIN line must be set low to ensure that the control register is not accidentally overwritten or the sequence operation is interrupted. If the control register is written to at any time during the sequence, the user must ensure that the SEQ1 and SEQ0 bits are set to 1, 0 to avoid interrupting the automatic conversion sequence. This pattern continues until the AD7923 is written to and the SEQ1 and SEQ0 bits are configured with any bit combination except 1, 0, resulting in the termination of the sequence. If uninterrupted, however (write bit = 0, or write bit = 1 and SEQ1 and SEQ0 bits are set to 1, 0), then upon completion of the sequence, the AD7923 sequencer returns to Channel 0 and commences the sequence again. NOTES 1. ALL UNUSED INPUT CHANNELS MUST BE CONNECTED TO AGND. Figure 19. Typical Connection Diagram Analog Input Selection Any one of four analog input channels can be selected for conversion by programming the multiplexer with Address Bits ADD1 and ADD0 in the control register. The channel configurations are shown in Table 7. The AD7923 can also be configured to automatically cycle through selected channels. The sequencer feature is accessed via the SEQ1 and SEQ0 bits in the control register (see Table 9). The AD7923 can be programmed to continuously convert on a number of consecutive channels in ascending order from Channel 0 to a selected final channel as determined by Channel Address Bits ADD1 and ADD0. This is possible if the SEQ1 and SEQ0 bits are set to 1, 1. The next serial transfer then acts on the sequence programmed by executing a conversion on Channel 0. The next serial transfer results in a conversion on Channel 1, and so on, until the channel selected via Address Bits ADD1 and ADD0 is reached. It is not necessary to write to Another advantage of SCLK, DIN, and CS not being restricted by the AVDD + 0.3 V limit is that possible power supply sequencing issues are avoided. If CS, DIN, or SCLK are applied before AVDD, there is no risk of latchup as there would be on the analog inputs if a signal greater than 0.3 V were applied prior to AVDD. VDRIVE The AD7923 also has the VDRIVE feature. VDRIVE controls the voltage at which the serial interface operates. VDRIVE allows the ADC to easily interface to both 3 V and 5 V processors. For example, if the AD7923 were operated with an AVDD of 5 V, the VDRIVE pin could be powered from a 3 V supply. The AD7923 has a larger dynamic range with an AVDD of 5 V while still being able to interface to 3 V processors. Care should be taken to ensure that VDRIVE does not exceed AVDD by more than 0.3 V (see the Absolute Maximum Ratings section). Reference An external reference source should be used to supply the 2.5 V reference to the AD7923. Errors in the reference source result in gain errors in the AD7923 transfer function and add to the specified full-scale errors of the part. A capacitor of at least 0.1 μF should be placed on the REFIN pin. Suitable reference sources for the AD7923 include the AD780, REF 192, and the AD1582. Rev. C | Page 16 of 24 AD7923 CS SCLK MODES OF OPERATION DOUT The AD7923 has a number of different modes of operation, which are designed to provide flexible power management options. These options can be chosen to optimize the power dissipation/throughput rate ratio for differing application requirements. The mode of operation of the AD7923 is controlled by the power management bits, PM1 and PM0, in the control register, as detailed in Table 8. When power supplies are first applied to the AD7923, care should be taken to ensure that the part is placed in the required mode of operation (see the Powering Up the AD7923 section). Normal Mode (PM1 = PM0 = 1) This mode is intended for the fastest throughput rate performance where the user does not have to worry about power-up time since the AD7923 remains fully powered at all times. Figure 20 shows the general diagram of the operation of the AD7923 in this mode. The conversion is initiated on the falling edge of CS and the track-and-hold enters hold mode, as described in the Serial Interface section. The data presented to the AD7923 on the DIN line during the first 12 clock cycles of the data transfer is loaded into the control register (provided the write bit is set to 1). The part remains fully powered up in normal mode at the end of the conversion as long as PM1 and PM0 are set to 1 in the write transfer during that same conversion. To ensure continued operation in normal mode, PM1 and PM0 must both be loaded with 1 on every data transfer, assuming a write operation is taking place. If the write bit is set to 0, the power management bits are left unchanged and the part remains in normal mode. Sixteen serial clock cycles are required to complete the conversion and access the conversion result. The track-and-hold go back into track on the 14th SCLK falling edge. CS may then idle high until the next conversion or may idle low until sometime prior to the next conversion (effectively idling CS low). For specified performance, the throughput rate should not exceed 200 kSPS, which means there should be no less than 5 μs between consecutive falling edges of CS when converting. The actual frequency of the SCLK used determines the duration of the conversion within this 5 μs cycle; however, once a conversion is complete, and CS has returned high, a minimum of the quiet time, tQUIET, must elapse before bringing CS low again to initiate another conversion. DIN 1 12 16 2 LEADING ZEROS + 2 CHANNEL IDENTIFIER BITS + CONVERSION RESULT DATA INTO CONTROL REGISTER CONTROL REGISTER DATA IS LOADED ON THE FIRST 12 SCLK CYCLES. 03086-020 If 2.5 V is applied to the REFIN pin, the analog input range can be either 0 V to 2.5 V or 0 V to 5 V, depending on the setting of the range bit in the control register. Figure 20. Normal Mode Operation Full Shutdown (PM1 = 1, PM0 = 0) In this mode, all internal circuitry on the AD7923 is powered down. The part retains information in the control register during full shutdown. The AD7923 remains in full shutdown until the power management bits in the control register, PM1 and PM0, are changed. If a write to the control register occurs while the part is in full shutdown, with the power management bits changed to PM0 = PM1 = 1, normal mode, the part begins to power up on the CS rising edge. The track-and-hold that was in hold while the part was in full shutdown returns to tracking on the 14th SCLK falling edge. A full 16-SCLK transfer must occur to ensure that the control register contents are updated; however, the DOUT line is not driven during this wake-up transfer. To ensure that the part is fully powered up, tPOWER UP (t12) should have elapsed before the next CS falling edge; otherwise invalid data is read if a conversion is initiated before this time. Figure 21 shows the general diagram for this sequence. Auto Shutdown (PM1 = 0, PM0 = 1) In this mode, the AD7923 automatically enters shutdown at the end of each conversion when the control register is updated. When the part is in shutdown, the track-and-hold is in hold mode. Figure 22 shows the general diagram of the operation of the AD7923 in this mode. In shutdown mode all internal circuitry on the AD7923 is powered down. The part retains information in the control register during shutdown. The AD7923 remains in shutdown until the next CS falling edge it receives. On this CS falling edge, the track-and-hold that was in hold while the part was in shutdown returns to tracking. Wake-up time from auto shutdown is 1 μs maximum, and the user should ensure that 1 μs has elapsed before attempting a valid conversion. When running the AD7923 with a 20 MHz clock, one dummy 16 SCLK transfer should be sufficient to ensure that the part is fully powered up. During this dummy transfer, the contents of the control register should remain unchanged, therefore the write bit should be 0 on the DIN line. Depending on the SCLK frequency used, this dummy transfer may affect the achievable throughput rate of the part, with every other data transfer being a valid conversion result. If, for example, the maximum SCLK frequency of 20 MHz is used, the auto shut-down mode could be used at the full throughout rate of 200 kSPS without affecting the throughput rate at all. Rev. C | Page 17 of 24 AD7923 If the desired mode of operation is full shutdown, then again only one dummy cycle is required after supplies are applied. In this dummy cycle, the user simply sets the power management bits, PM1, PM0 = 1, 0, and upon the rising edge of CS at the end of that serial transfer, the part enters full shutdown. If the desired mode of operation is auto shutdown after supplies are applied, two dummy cycles are required, the first with DIN tied high and the second dummy cycle to set the power management bits PM1 and PM0 = 0,1. On the second CS rising edge after the supplies are applied, the control register contains the correct information and the part enters auto shutdown mode as programmed. If power consumption is of critical concern, then in the first dummy cycle the user may set PM1, PM0 = 1, 0, that is, full shutdown, and then place the part into auto shutdown in the second dummy cycle. For illustration purposes, Figure 25 is shown with DIN tied high on the first dummy cycle in this case. Only a portion of the cycle time is taken up by the conversion time and the dummy transfer for wakeup. In this mode, the power con-sumption of the part is greatly reduced because the part enters shutdown at the end of each conversion. When the control register is programmed to move into auto shutdown, it does so at the end of the conversion. The user can move the ADC in and out of the low power state by controlling the CS signal. POWERING UP THE AD7923 When supplies are first applied to the AD7923, the ADC can power up in any of the operating modes of the part. To ensure that the part is placed into the required operating mode, the user should perform a dummy cycle operation, as outlined in Figure 23 through Figure 25. The dummy conversion operation must be performed to place the part into the desired mode of operation. To ensure that the part is in normal mode, this dummy cycle operation can be performed with the DIN line tied high, that is, PM1, PM0 = 1, 1 (depending on other required settings in the control register), but the minimum power-up time of 1 μs must be allowed from the rising edge of CS, where the control register is updated, before attempting the first valid conversion. This is to allow for the possibility that the part initially powered up in shutdown. PART IS IN FULL SHUTDOWN Figure 23, Figure 24, and Figure 25 each show the required dummy cycle(s) after supplies are applied in the case of normal mode, full shutdown mode, and auto shutdown mode, respectively, being the desired mode of operation. PART BEGINS TO POWER UP ON CS RISING EDGE AS PM1 = PM0 =1 THE PART IS FULLY POWERED UP ONCE tPOWER UP HAS ELAPSED t12 CS 1 14 16 1 14 16 SCLK DOUT CHANNEL IDENTIFIER BITS + CONVERSION RESULT DATA INTO CONTROL REGISTER CONTROL REGISTER IS LOADED ON THE FIRST 12 CLOCKS. PM1 = 1, PM0 = 1 03086-021 DATA INTO CONTROL REGISTER DIN TO KEEP THE PART IN NORMAL MODE, LOAD PM1 = PM0 = 1 IN CONTROL REGISTER Figure 21. Full Shutdown Mode Operation PART ENTERS SHUTDOWN ON CS RISING EDGE AS PM1 = 0, PM0 =1 PART BEGINS TO POWER UP ON CS FALLING EDGE CS DOUT DIN DUMMY CONVERSION 1 12 16 CHANNEL IDENTIFIER BITS + CONVERSION RESULT DATA INTO CONTROL REGISTER CONTROL REGISTER IS LOADED ON THE FIRST 12 CLOCKS, PM1 = 0, PM0 = 1 1 12 16 INVALID DATA DATA INTO CONTROL REGISTER CONTROL REGISTER CONTENTS SHOULD NOT CHANGE, WRITE BIT = 0 Figure 22. Auto Shutdown Mode Operation Rev. C | Page 18 of 24 1 12 16 CHANNEL IDENTIFIER BITS + CONVERSION RESULT DATA INTO CONTROL REGISTER TO KEEP PART IN THIS MODE, LOAD PM1 = 0, PM0 = 1 IN CONTROL REGISTER OR SET WRITE BIT = 0 03086-022 SCLK PART IS FULLY POWERED UP PART ENTERS SHUTDOWN ON CS RISING EDGE AS PM1 = 0, PM0 =1 AD7923 IF IN SHUTDOWN AT POWER-ON PART BEGINS TO POWER UP ON CS RISING EDGE AS PM1 = PM0 = 1 PART IS IN UNKNOWN MODE AFTER POWER-ON ALLOW tPOWER TO ELAPSE t12 CS 1 14 16 1 14 16 SCLK DOUT INVALID DATA CHANNEL IDENTIFIER BITS + CONVERSION RESULT DIN 03086-023 DATA INTO CONTROL REGISTER DIN LINE HIGH FOR FIRST DUMMY CONVERSION TO KEEP THE PART IN NORMAL MODE, LOAD PM1 = PM0 = 1 IN CONTROL REGISTER Figure 23. Placing the AD7923 into Normal Mode after Supplies are First Applied PART IS IN UNKNOWN MODE AFTER POWER-ON PART ENTERS SHUTDOWN ON CS RISING EDGE AS PM1 = PM0 = 0 CS 1 14 16 SCLK INVALID DATA DOUT 03086-024 DATA INTO CONTROL REGISTER DIN CONTROL REGISTER IS LOADED ON THE FIRST 12 CLOCKS. PM1 = 1, PM0 = 0 Figure 24. Placing the AD7923 into Full Shutdown Mode after Supplies are First Applied PART IS IN UNKNOWN MODE AFTER POWER-ON PART ENTERS AUTO SHUTDOWN ON CS RISING EDGE AS PM1 = 0, PM0 = 1 CS 1 14 16 1 14 16 SCLK INVALID DATA INVALID DATA DIN DATA INTO CONTROL REGISTER DIN LINE HIGH FOR FIRST DUMMY CONVERSION CONTROL REGISTER IS LOADED ON THE FIRST 12 CLOCKS. PM1 = 0, PM0 = 1 03086-025 DOUT Figure 25. Placing the AD7923 into Auto Shutdown Mode after Supplies are First Applied POWER vs. THROUGHPUT RATE In auto shutdown mode, the average power consumption of the ADC can be reduced at any given throughput rate. The power saving depends on the SCLK frequency used, that is, conversion time. In some cases where the conversion time is a large proportion of the cycle time, the throughput rate needs to be reduced to take advantage of the power-down modes. Assuming a 20 MHz SCLK is used, the conversion time is 800 ns, but the cycle time is 5 μs when the sampling rate is at a maximum of 200 kSPS. If the AD7923 is placed into shutdown for the remainder of the cycle time, then on average far less power is consumed in every cycle compared to leaving the device in normal mode. Furthermore, Figure 26 shows how, as the throughput rate is reduced, the part remains in its shutdown longer and the average power consumption drops accordingly over time. Rev. C | Page 19 of 24 AD7923 For example, if the AD7923 is operated in a continuous sampling mode, with a throughput rate of 200 kSPS and an SCLK of 20 MHz (AVDD = 5 V), and the device is placed in auto shutdown mode, that is, if PM1 = 0 and PM0 = 1, then the power consumption is calculated as follows: The maximum power dissipation during conversion is 13.5 mW (IDD = 2.7 mA max, AVDD = 5 V). If the power-up time from auto shutdown is one dummy cycle, that is 1 μs, and the remaining conversion time is another cycle, that is, 800 ns, then the AD7923 can be said to dissipate 13.5 mW for 1.8 μs during each conversion cycle. For the remainder of the conversion cycle, 3.2 μs, the part remains in shutdown. The AD7923 can be said to dissipate 2.5 μW for the remaining 3.2 μs of the conver-sion cycle. If the throughput rate is 200 kSPS, the cycle time is 5 μs and the average power dissipated during each cycle is (1.8/5) × (13.5 mW) + (3.2/5) × (2.5 μW) = 4.8616 mW. Figure 26 shows the maximum power vs. throughput rate when using the auto shutdown mode with 5 V and 3 V supplies. Sixteen serial clock cycles are required to perform the conversion process and to access data from the AD7923. For the AD7923, the 12 bits of data are preceded by two leading 0s and Channel Address Bits ADD1 and ADD0, identifying which channel the result corresponds to. CS going low clocks out the first leading 0 to be read by the microcontroller or DSP on the first falling edge of SCLK. The first falling edge of SCLK also clocks out the second leading 0 to be read by the microcontroller or DSP on the second SCLK falling edge, and so on. The remaining two address bits and 12 data bits are then clocked out by subsequent SCLK falling edges, beginning with the first Address Bit ADD1, thus the second falling clock edge on the serial clock has the second leading 0 and also clocks out Address Bit ADD1. The final bit in the data transfer is valid on the 16th falling edge, having been clocked out on the previous (15th) falling edge. Writing information to the control register takes place on the first 12 falling edges of SCLK in a data transfer, assuming the MSB, that is, the write bit, has been set to 1. 10 The 16-bit word read from the AD7923 always contain two leading 0s, two channel address bits that the conversion result corresponds to, followed by the 12-bit conversion result. AVDD = 5V AVDD = 3V 1 POWER (mW) Writing Between Conversions 03086-026 0.1 0.01 0 20 40 60 80 100 120 140 THROUGHPUT (kSPS) 160 180 100 Figure 26. Power vs. Throughput Rate SERIAL INTERFACE Figure 27 shows the detailed timing diagrams for serial interfacing to the AD7923. The serial clock provides the conversion clock and controls the transfer of information to and from the AD7923 during each conversion. The CS signal initiates the data transfer and conversion process. The falling edge of CS puts the track-and-hold into hold mode and takes the bus out of three-state; the analog input is sampled at this point. The conversion is also initiated at this point and requires 16 SCLK cycles to complete. The track-and-hold returns to track mode at Point B on the 14th SCLK falling edge, as shown in Figure 27. On the 16th SCLK falling edge the DOUT line returns to three-state. If the rising edge of CS occurs before 16 SCLKs have elapsed, the conversion is terminated, the DOUT line returns to three-state, and the control register is not updated; otherwise DOUT returns to three-state on the 16th SCLK falling edge, as shown in Figure 27. As outlined in the operating modes section, not less than 5 μs should be left between consecutive valid conversions. There is one exception, however: consider the case when writing to the AD7923 to power it up from shutdown prior to a valid conversion. The user must write to the part to tell it to power up before it can convert successfully. Once the serial write to power up has finished, the user might want to perform the conversion as soon as possible without waiting an additional 5 μs before bringing CS low for the conversion. In this case, as long as there is a minimum of 5 μs between each valid conversion, only the quiet time between the CS rising edge at the end of the write to power up and the next CS falling edge needs to be met. Figure 28 illustrates this point. Note that when writing to the AD7923 between these valid conversions, the DOUT line is not driven during the extra write operation. It is critical that an extra write operation as outlined above is never issued between valid conversions when the AD7923 is executing a sequence function, because the falling edge of CS in the extra write moves the mux to the next channel in the sequence. This means that when the next valid conversion takes place a channel result would be missed. Rev. C | Page 20 of 24 AD7923 CS 2 3 t3 5 ZERO THREESTATE ZERO WRITE ADD1 ADD0 DB11 SEQ1 DONTC B 11 12 13 14 DONTC 15 16 t5 t11 t8 DB10 2 IDENTIFICATION BITS t9 6 t7 t4 DOUT DIN 4 DB4 DB3 DB2 DB1 THREESTATE t10 ADD1 tQUIET DB0 ADD0 CODING DONTC DONTC DONTC 03086-027 1 SCLK tCONVERT t6 t2 DONTC Figure 27. Serial Interface Timing Diagram tCYCLE 5s MIN tQUIET MIN CS 1 16 1 16 1 16 DOUT VALID DATA VALID DATA DIN POWER-UP 03086-028 SCLK Figure 28. General Timing Diagram MICROPROCESSOR INTERFACING The serial interface on the AD7923 allows the part to be directly connected to a range of many different microprocessors. This section explains how to interface the AD7923 with some of the more common microcontroller and DSP serial interface protocols. SCLK CLKX CLKR DOUT DR DIN DT CS FSX VDRIVE FSR 1 ADDITIONAL PINS REMOVED FOR CLARITY. VDD 03086-029 AD7923-to-TMS320C541 The serial interface on the TMS320C541 uses a continuous serial clock and frame synchronization signals to synchronize the data transfer operations with peripheral devices like the AD7923. The CS input allows easy interfacing between the TMS320C541 and the AD7923 without any glue logic required. The serial port of the TMS320C541 is set up to operate in burst mode with internal CLKX0 (Tx serial clock on Serial Port 0) and FSX0 (Tx frame sync from Serial Port 0). The serial port control register (SPC) must have the following setup: FO = 0, FSM = 1, MCM = 1, and TXM = 1. The connection diagram is shown in Figure 29. It should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the TMS320C541 provides equidistant sampling. The VDRIVE pin of the AD7923 takes the same supply voltage as the TMS320C541. This allows the ADC to operate at a higher voltage than the serial interface, that is, the TMS320C541, if necessary. TMS320C5411 AD79231 Figure 29. Interfacing to the TMS320C541 AD7923-to-ADSP-21xx The ADSP-21xx family of DSPs is interfaced directly to the AD7923 without any glue logic required. The VDRIVE pin of the AD7923 takes the same supply voltage as the ADSP-218x, which allows the ADC to operate at a higher voltage than the serial interface, that is, ADSP-218x, if necessary. The SPORT0 control register should be set up as follows: TFSW = RFSW = 1, alternate framing INVRFS = INVTFS = 1, active low frame signal DTYPE = 00, right justify data SLEN = 1111, 16-bit data-words ISCLK = 1, internal serial clock TFSR = RFSR = 1, frame every word IRFS = 0 ITFS = 1 Rev. C | Page 21 of 24 AD7923 ADSP-218x1 AD79231 SCLK SCLK DOUT DR CS RFS TFS DT 1 ADDITIONAL PINS REMOVED FOR CLARITY. VDD 03086-030 VDRIVE DIN Figure 30. Interfacing to the ADSP-218x The timer register, for instance, is loaded with a value that provides an interrupt at the required sample interval. When an interrupt is received, a value is transmitted with TFS/DT (ADC control word). The TFS is used to control the RFS and therefore the reading of data. The frequency of the serial clock is set in the SCLKDIV register. When the instruction to transmit with TFS is given (that is, AX0 = TX0), the state of the SCLK is checked. The DSP waits until the SCLK has gone high, low, and high before the transmission starts. If the timer and SCLK values are chosen such that the instruction to transmit occurs on or near the rising edge of SCLK, the data can be transmitted, or it can wait until the next clock edge. For example, if the ADSP-2189 has a 20 MHz crystal such that it has a master clock frequency of 40 MHz, then the master cycle time is 25 ns. If the SCLKDIV register is loaded with the value 3, then a SCLK of 5 MHz is obtained, and eight master clock periods elapse for every SCLK period. Depending on the throughput rate selected, if the timer registers are loaded with the value 803, 100.5 SCLKs occur between interrupts and subsequently between transmit instructions. This situation results in nonequidistant sampling since the transmit instruction occurs on a SCLK edge. If the number of SCLKs between interrupts is an integer of N, equidistant sampling is implemented by the DSP. AD7923-to-DSP563xx The connection diagram in Figure 31 shows how the AD7923 can be connected to the synchronous serial interface (ESSI) of the DSP563xx family of DSPs from Motorola. Each ESSI (two on board) is operated in synchronous mode (SYN bit in CRB = 1), with an internally generated word length frame sync for both Tx and Rx (bits FSL1 = 0 and FSL0 = 0 in CRB). Normal operation of the ESSI is selected by making MOD = 0 in the CRB. Set the word length to 16 by setting bits WL1 = 1 and WL0 = 0 in CRA. The FSP bit in the CRB should be set to 1 so the frame sync is negative. It should be noted that for signal processing applications, it is imperative that the frame synchronization signal from the DSP563xx provides equidistant sampling. In the example shown in Figure 31, the serial clock is taken from the ESSI, therefore the SCK0 pin must be set as an output, SCKD = 1. The VDRIVE pin of the AD7923 takes the same supply voltage as the DSP563xx, which allows the ADC to operate at a higher voltage than the serial interface, that is, DSP563xx, if necessary. DSP563xx1 AD79231 SCLK SCK DOUT SRD DIN STD CS SC2 VDRIVE 1 ADDITIONAL Rev. C | Page 22 of 24 PINS REMOVED FOR CLARITY. VDD Figure 31. Interfacing to the DSP563xx 03086-031 The connection diagram is shown in Figure 30. The ADSP-218x has the TFS and RFS of the SPORT tied together, with TFS set as an output and RFS set as an input. The DSP operates in alternate framing mode and the SPORT control register is set up as described. The frame synchronization signal generated on the TFS is tied to CS and, as with all signal processing applications, equidistant sampling is necessary. However, in this example, the timer interrupt is used to control the sampling rate of the ADC, and under certain conditions equidistant sampling might not be achieved. AD7923 APPLICATION HINTS GROUNDING AND LAYOUT The AD7923 has very good immunity to noise on the power supplies as can be seen by the PSRR vs. supply ripple frequency plot, Figure 6. However, care should still be taken in grounding and layout. The printed circuit board that houses the AD7923 should be designed such that the analog and digital sections are separated and confined to certain areas of the board. This facilitates the use of ground planes that can be separated easily. A minimum etch technique is generally best for ground planes since it gives the best shielding. All three AGND pins of the AD7923 should be sunk into the AGND plane. Digital and analog ground planes should be joined at only one place. If the AD7923 is in a system where multiple devices require an AGND to DGND connection, the connection should still be made at one point only, a star ground point that should be established as close as possible to the AD7923. Avoid running digital lines under the device since they couple noise onto the die. The analog ground plane should be allowed to run under the AD7923 to avoid noise coupling. The power supply lines to the AD7923 should use as large a trace as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals, like clocks, should be shielded with digital ground to avoid radiating noise to other sections of the board, and clock signals should never be run near the analog inputs. Avoid crossover of digital and analog signals. Traces on opposite sides of the board should run at right angles to each other. This reduces the effects of feedthrough through the board. A microstrip technique is by far the best technique, but is not always possible with a doublesided board. In this technique, the component side of the board is dedicated to ground planes, while signals are placed on the solder side. Good decoupling is also important. All analog supplies should be decoupled with 10 μF tantalum in parallel with 0.1 μF capacitors to AGND. To achieve the best results from these decoupling components, they must be placed as close as possible to the device, ideally right up against the device. The 0.1 μF capacitors should have low effective series resistance (ESR) and low effective series inductance (ESI), such as the common ceramic types or surface mount types, which provide a low impedance path to ground at high frequencies to handle transient currents from internal logic switching. EVALUATING THE AD7923 PERFORMANCE The recommended layout for the AD7923 is outlined in the evaluation board for the AD7923. The evaluation board package includes a fully assembled and tested evaluation board, documentation, and software for controlling the board from the PC via the EVAL-CONTROL BRD2. The EVAL-CONTROL BRD2 can be used with the AD7923 evaluation board and many other Analog Devices evaluation boards ending in the CB designator to demonstrate and evaluate the ac and dc performance of the AD7923. The software allows the user to perform ac (fast Fourier transform) and dc (histogram of codes) tests on the AD7923. The software and documentation are on a CD shipped with the evaluation board. Rev. C | Page 23 of 24 AD7923 OUTLINE DIMENSIONS 5.10 5.00 4.90 16 9 4.50 4.40 4.30 6.40 BSC 1 8 PIN 1 1.20 MAX 0.15 0.05 0.20 0.09 0.30 0.19 0.65 BSC COPLANARITY 0.10 SEATING PLANE 0.75 0.60 0.45 8° 0° COMPLIANT TO JEDEC STANDARDS MO-153-AB Figure 32. 16-Lead Thin Shrink Small Outline Package [TSSOP] (RU-16) Dimensions shown in millimeters ORDERING GUIDE Model1, 2, 3, 4 AD7923BRU AD7923BRUZ AD7923BRUZ-REEL AD7923BRUZ-REEL7 AD7923WYRUZ-REEL7 EVAL-AD7923CBZ EVAL-CONTROL BRD2 Temperature Range −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C Linearity Error (LSB)5 ±1 ±1 ±1 ±1 ±1 Package Option RU-16 RU-16 RU-16 RU-16 RU-16 Package Description 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP 16-Lead TSSOP Evaluation Board Controller Board 1 Z = RoHS Compliant Part. W = Qualified for Automotive Applications. The EVAL-AD7923CBZ can be used as a standalone evaluation board or in conjunction with the evaluation controller board for evaluation/demonstration purposes. 4 The EVAL-CONTROL BRD2 is a complete unit allowing a PC to control and communicate with all Analog Devices evaluation boards ending in the CB designators. To order a complete evaluation kit, order the particular ADC evaluation board, for example, the EVAL-AD7923CB, the EVAL-CONTROL BRD2, and a 12 V ac transformer. See the relevant evaluation board application note for more information. 5 Linearity error refers to integral linearity error. 2 3 AUTOMOTIVE PRODUCTS The AD7923W models are available with controlled manufacturing to support the quality and reliability requirements of automotive applications. Note that these automotive models may have specifications that differ from the commercial models; therefore, designers should review the Specifications section of this data sheet carefully. Only the automotive grade products shown are available for use in automotive applications. Contact your local Analog Devices account representative for specific product ordering information and to obtain the specific Automotive Reliability reports for these models. ©2002–2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D03086-0-5/11(C) Rev. C | Page 24 of 24