a Single and Multichannel, Synchronous Voltage-to-Frequency Converters AD7741/AD7742 FEATURES AD7741: One Single-Ended Input Channel AD7742: Two Differential or Three Pseudo-Differential Input Channels Integral Nonlinearity of 0.012% at fOUT(Max) = 2.75 MHz (AD7742) and at fOUT(Max) = 1.35 MHz (AD7741) Single +5 V Supply Operation Buffered Inputs Programmable Gain Analog Front-End On-Chip +2.5 V Reference Internal/External Reference Option Power Down to 35 A Max Minimal External Components Required 8-Lead and 16-Lead DIP and SOIC Packages FUNCTIONAL BLOCK DIAGRAMS VDD PD REFIN/OUT +2.5V REFERENCE VOLTAGE-TOFREQUENCY MODULATOR X1 VIN fOUT AD7741 CLOCK GENERATION CLKIN APPLICATIONS Low Cost Analog-to-Digital Conversion Signal Isolation CLKOUT VDD GND PD UNI/BIP GAIN AD7742 VIN1 VIN2 VIN3 GENERAL DESCRIPTION The AD7741/AD7742 are a new generation of synchronous voltage-to-frequency converters (VFCs). The AD7741 is a single-channel version in an 8-lead package (SOIC/DIP) and the AD7742 is a multichannel version in a 16-lead package (SOIC/ DIP). No user trimming is required to achieve the specified performance. The AD7741 has a single buffered input whereas the AD7742 has four buffered inputs that may be configured as two fullydifferential inputs or three pseudo-differential inputs. Both parts include an on-chip +2.5 V bandgap reference that provides the user with the option of using this internal reference or an external reference. INPUT MUX POWER-DOWN LOGIC X1/X2 POWER-DOWN LOGIC VOLTAGE-TOFREQUENCY MODULATOR fOUT VIN4 +2.5V REFERENCE CLOCK GENERATION A1 A0 GND CLKIN CLKOUT REFIN REFOUT The AD7741 has a single-ended voltage input range from 0 V to REFIN. The AD7742 has a differential voltage input range from –VREF to +VREF. Both parts operate from a single +5 V supply consuming typically 6 mA, and also contain a powerdown feature that reduces the current consumption to less than 35 µA. REV. 0 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 which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 1999 (VDD = +4.75 V to +5.25 V; VREF = +2.5 V; fCLKIN = 6.144 MHz; all specifications TMIN to MAX unless otherwise noted.) AD7741–SPECIFICATIONS T 2 Parameter DC PERFORMANCE Integral Nonlinearity fCLKIN = 200 kHz3 fCLKIN = 3 MHz3 fCLKIN = 6.144 MHz Offset Error Gain Error Offset Error Drift3 Gain Error Drift3 Power Supply Rejection Ratio3 ANALOG INPUT5 Input Current Input Voltage Range +2.5 V REFERENCE (REFIN/OUT) REFIN Nominal Input Voltage Input Impedance6 REFOUT Output Voltage Output Impedance3 Reference Drift3 Line Rejection Reference Noise (0.1 Hz to 10 Hz)3 LOGIC OUTPUT Output High Voltage, VOH Output Low Voltage, VOL Minimum Output Frequency Maximum Output Frequency LOGIC INPUT PD ONLY Input High Voltage, VIH Input Low Voltage, VIL Input Current Pin Capacitance CLKIN ONLY Input High Voltage, VIH Input Low Voltage, VIL Input Current Pin Capacitance B and Y Version1 Min Typ Max 0 +0.8 ± 30 ± 16 –63 ± 50 0 ± 100 VREF 2.5 N/A 2.38 2.50 1 ± 50 –60 100 % of Span4 % of Span % of Span mV % of Span µV/°C ppm of Span/°C dB 2.60 0.4 0.05 fCLKIN 0.45 fCLKIN 2.4 ∆VDD = ± 5% V kΩ ppm/°C dB µV p-p V V Hz Hz 6 V V nA pF 6 0.8 ±2 10 V V µA pF 6.144 MHz 5.25 8 35 V mA µA µs 3.5 15 30 VDD > 4.8 V nA V 0.8 ± 100 10 4.75 Conditions/Comments V 4.0 CLOCK FREQUENCY Input Frequency POWER REQUIREMENTS VDD IDD (Normal Mode) IDD (Power-Down) Power-Up Time3 ± 0.012 ± 0.012 ± 0.024 ± 40 +1.6 Units Output Sourcing 800 µA7 Output Sinking 1.6 mA7 VIN = 0 V VIN = VREF For Specified Performance Output Unloaded Coming Out of Power-Down Mode NOTES 1 Temperature ranges: B Version –40°C to +85°C: Y Version: –40°C to +105°C. 2 See Terminology. 3 Guaranteed by design and characterization, not production tested. 4 Span = Maximum Output Frequency–Minimum Output Frequency. 5 The absolute voltage on the input pin must not go more positive than V DD – 2.25 V or more negative than GND. 6 Because this pin is bidirectional, any external reference must be capable of sinking/sourcing 400 µA in order to overdrive the internal reference. 7 These logic levels apply to CLKOUT only when it is loaded with one CMOS load. Specifications subject to change without notice. –2– REV. 0 AD7741/AD7742 AD7742–SPECIFICATIONS (VT DD MAX = +4.75 V to +5.25 V; VREF = +2.5 V; fCLKIN = 6.144 MHz; all specifications TMIN to unless otherwise noted.) B Version1 Parameter3 Min Typ DC PERFORMANCE Integral Nonlinearity fCLKIN = 200 kHz4 fCLKIN = 3 MHz4 fCLKIN = 6.144 MHz Offset Error Gain Error +0.2 +0.2 Offset Error Drift 4 Gain Error Drift 4 Power Supply Rejection Ratio 4 Channel-to-Channel Isolation 4 Common-Mode Rejection ANALOG INPUTS (VIN1–VIN4)6 Input Current Common-Mode Input Range Differential Input Range VOLTAGE REFERENCE REFIN Nominal Input Voltage Input Impedance 4 fCLKIN = 3 MHz fCLKIN = 6.144 MHz REFOUT Output Voltage Output Impedance 4 Reference Drift 4 Line Rejection Reference Noise (0.1 Hz to 10 Hz) 4 LOGIC OUTPUT Output High Voltage, V OH Output Low Voltage, VOL Minimum Output Frequency –60 ± 50 +0.5 –VREF/Gain 0 +0.2 +0.2 –58 ± 100 VDD – 1.75 +0.5 +VREF/Gain –VREF/Gain +VREF/Gain 0 2.38 Typ +1.2 +1.2 ± 12 ± 12 ±2 ±4 –70 –75 –78 ± 50 Max Units ± 0.015 ± 0.015 ± 0.015 ± 40 ± 40 +2.2 +2.2 % of Span 5 % of Span % of Span mV mV % of Span % of Span µV/°C µV/°C ppm of Span/°C ppm of Span/°C dB dB dB ± 100 VDD – 1.75 +VREF/Gain +VREF/Gain nA V V V 2.5 70 35 2.50 1 ± 50 –70 2.60 2.38 100 2.50 1 ± 50 –70 2.60 0.05 fCLKIN 0.05 fCLKIN 0.45 fCLKIN 0.45 fCLKIN Hz 0.4 2.4 6 6 0.8 ±2 10 3.5 6 0.8 ± 100 10 V V nA pF 6 0.8 ±2 10 V V µA pF 6.144 MHz 5.25 8 35 V mA µA µs 3.5 6.144 4.75 6 25 30 5.25 8 35 4.75 6 25 30 NOTES 1 Temperature range: B Version: –40°C to +85°C. Temperature range: Y Version: –40°C to +105°C. See Terminology. 4 Guaranteed by design and characterization, not production tested. 5 Span = Maximum Output Frequency–Minimum Output Frequency. 6 The absolute voltage on the input pins must not go more positive than V DD – 1.75 V or more negative than +0.5 V. 7 These logic levels apply to CLKOUT only when it is loaded with one CMOS load. 2 3 Specifications subject to change without notice . REV. 0 V kΩ ppm/°C dB V V Hz 0.8 ± 100 10 –3– Bipolar Mode Unipolar Mode µV p-p 4.0 0.4 2.4 Unipolar Mode Bipolar Mode Unipolar Mode Bipolar Mode Unipolar Mode Bipolar Mode Unipolar Mode Bipolar Mode ∆VDD = ± 5% kΩ kΩ 100 4.0 Conditions/Comments V 70 35 CLOCK FREQUENCY Input Frequency POWER REQUIREMENTS VDD IDD (Normal Mode) IDD (Power-Down) Power-Up Time 4 ± 0.0122 ± 0.0122 ± 0.0122 ± 40 ± 40 +2.2 +2.2 Min 2.5 Maximum Output Frequency LOGIC INPUT ALL EXCEPT CLKIN Input High Voltage, V IH Input Low Voltage, V IL Input Current Pin Capacitance CLKIN ONLY Input High Voltage, V IH Input Low Voltage, V IL Input Current Pin Capacitance +1.2 +1.2 ± 12 ± 12 ±2 ±4 –70 –75 –78 Y Version2 Max Output Sourcing 800 µA7 Output Sinking 1.6 mA7 VIN = 0 V (Unipolar), VIN = –VREF/Gain (Bipolar) VIN = VREF/Gain (Unipolar and Bipolar) For Specified Performance Output Unloaded Coming Out of PowerDown Mode AD7741/AD7742 TIMING CHARACTERISTICS1, 2, 3 (V Parameter fCLKIN tHIGH/tLOW t1 t2 t3 t4 DD = +4.75 V to +5.25 V; VREF = +2.5 V. All specifications TMIN to TMAX unless otherwise noted.) Limit at TMIN, TMAX (B and Y Version) Units 6.144 55/45 45/55 9 4 4 tHIGH ± 5 MHz max max min ns typ ns typ ns typ ns typ Conditions/Comments Input Clock Mark/Space Ratio fCLOCK Rising Edge to fOUT Rising Edge fOUT Rise Time fOUT Fall Time fOUT Pulsewidth NOTES 1 Guaranteed by design and characterization, not production tested. 2 All input signals are specified with tr = tf = 5 ns (10% to 90% of V DD) and timed from a voltage level of (V IL + VIH)/2. 3 See Figure 1. Specifications subject to change without notice. ABSOLUTE MAXIMUM RATINGS 1, 2 tHIGH (TA = +25°C unless otherwise noted) VDD to GND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3␣ V to +7 V Analog Input Voltage to GND . . . . . . . . –5␣ V to VDD + 0.3 V Digital Input Voltage to GND . . . . . . . –0.3␣ V to VDD + 0.3 V Reference Input Voltage to GND . . . . –0.3 V to VDD + 0.3 V fOUT to GND . . . . . . . . . . . . . . . . . . . . –0.3 V to VDD + 0.3 V Operating Temperature Range Automotive (Y Version) . . . . . . . . . . . . . . –40°C to +105°C Industrial (B Version) . . . . . . . . . . . . . . . . –40°C to +85°C Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150°C Plastic DIP Package Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . 450 mW θJA Thermal Impedance (8 Lead) . . . . . . . . . . . . . 125°C/W θJA Thermal Impedance (16 Lead) . . . . . . . . . . . . 117°C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . +215°C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220°C SOIC Package Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . 450 mW θJA Thermal Impedance (8 Lead) . . . . . . . . . . . . . 157°C/W θJA Thermal Impedance (16 Lead) . . . . . . . . . . . . 125°C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . +215°C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220°C CLKIN t4 fOUT t2 t1 t3 Figure 1. Timing Diagram ORDERING GUIDE Models Temperature Ranges Package Descriptions Package Options AD7741BN AD7741BR AD7741YR AD7742BN AD7742BR AD7742YR –40°C to +85°C –40°C to +85°C –40°C to +105°C –40°C to +85°C –40°C to +85°C –40°C to +105°C Plastic DIP Small Outline Small Outline Plastic DIP Small Outline Small Outline N-8 R-8 R-8 N-16 R-16A R-16A NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 Transient currents of up to 100 mA will not cause SCR latch-up. 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 the AD7741/AD7742 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. –4– WARNING! ESD SENSITIVE DEVICE REV. 0 AD7741/AD7742 AD7741 PIN FUNCTION DESCRIPTION Pin No. Mnemonic Function 1 VDD 2 3 GND CLKOUT 4 CLKIN 5 REFIN/OUT 6 VIN 7 PD 8 fOUT Power Supply Input. These parts can be operated from +4.75 V to +5.25 V and the supply should be adequately decoupled to GND. Ground reference point for all circuitry on the part. External Clock Output. When the master clock for the device is a crystal, the crystal is connected between CLKIN and CLKOUT. When an external clock is applied to CLKIN, the CLKOUT pin provides an inverted clock signal. This clock should be buffered if it is to be used as a clock source elsewhere in the system. External Clock Input. The master clock for the device can be provided in the form of a crystal or an external clock. A crystal may be tied across the CLKIN and CLKOUT pins. Alternatively, the CLKIN pin may be driven by a CMOS-compatible clock and CLKOUT left unconnected. The frequency of the master clock may be as high as 6 MHz. This is the reference input to the core of the VFC and defines the span of the VFC. If this pin is left unconnected, the internal 2.5 V reference is used. Alternatively, a precision external reference (e.g., REF192) may be used to overdrive the internal reference. The internal bandgap reference has a high output impedance in order to allow it to be overdriven. The analog input to the VFC. It has an input range from 0 V to VREF. This input is buffered so it draws virtually no current from whatever source is driving it. Active Low Power-Down pin. When this input is low, the part enters power-down mode where it typically consumes 15 µA of current. Frequency Output. This pin provides the output of the synchronous VFC. PIN CONFIGURATION VDD 1 8 AD7741 fOUT PD TOP VIEW CLKOUT 3 (Not to Scale) 6 VIN GND 2 CLKIN 4 REV. 0 7 5 –5– REFIN/OUT AD7741/AD7742 AD7742 PIN FUNCTION DESCRIPTION Pin No. Mnemonic Function 1 2 fOUT VDD 3 4–5 6 GND A1, A0 CLKOUT 7 CLKIN 8 UNI/BIP 9 REFOUT 10 REFIN 11 VIN1 12 VIN2 13 VIN3 14 VIN4 15 16 GAIN PD Frequency Output. This pin provides the output of the synchronous VFC. Power Supply Input. These parts can be operated from +4.75 V to +5.25 V and the supply should be adequately decoupled to GND. Ground reference point for all circuitry on the part. Address Inputs used to select the input channel configuration. External Clock Output. When the master clock for the device is a crystal, the crystal is connected between CLKIN and CLKOUT. When an external clock is applied to CLKIN, the CLKOUT pin provides an inverted clock signal. This clock should be buffered if it is to be used as a clock source elsewhere in the system. External Clock Input. The master clock for the device can be provided in the form of a crystal or an external clock. A crystal may be tied across the CLKIN and CLKOUT pins. Alternatively, the CLKIN pin may be driven by a CMOS-compatible clock and CLKOUT left unconnected. The frequency of the master clock may be as high as 6 MHz. Control input which determines whether the device operates with differential bipolar analog input signals or differential unipolar analog input signals. 2.5 V Voltage Reference Output. This can be tied directly to REFIN. It may also be used as a reference to other parts of the system provided it is buffered first. This is the Reference Input to the core of the VFC and defines the span of the VFC. A 2.5 V reference is required at this pin. This may be provided by connecting it directly to REFOUT or by using a precision external reference (e.g., REF192). Buffered Analog Input Channel 1. This is either a pseudo-differential input with respect to VIN4 or it is the positive input of a truly-differential input pair with respect to VIN2. Buffered Analog Input Channel 2. This is either a pseudo-differential input with respect to VIN4 or it is the negative input of a truly-differential input pair with respect to VIN1. Buffered Analog Input Channel 3. This is the positive input of a truly-differential input pair with respect to VIN4. Buffered Analog Input Channel 4. This is either the common for pseudo-differential input with respect to VIN1 or VIN2 or it is the negative input of a truly-differential input pair with respect to VIN3. Gain Select input that controls whether the gain on the analog front-end is X1 or X2. Active Low Power-Down pin. When this input is low, the part enters power-down mode where it typically consumes 25 µA of current. PIN CONFIGURATION fOUT 1 16 PD VDD 2 15 GAIN GND 3 A1 4 14 VIN4 AD7742 13 VIN3 TOP VIEW A0 5 (Not to Scale) 12 VIN2 CLKOUT 6 11 VIN1 CLKIN 7 10 REFIN UNI/BIP 8 9 –6– REFOUT REV. 0 AD7741/AD7742 TERMINOLOGY GENERAL DESCRIPTION INTEGRAL NONLINEARITY The AD7741/AD7742 are a new generation of CMOS synchronous Voltage-to-Frequency Converters (VFCs) that use a charge-balance conversion technique. The AD7741 is a singlechannel version and the AD7742 is a multichannel version. The input voltage signal is applied to a proprietary programmable gain front-end based around an analog modulator that converts the input voltage into an output pulse train. For the VFC, Integral Nonlinearity (INL) is a measure of the maximum deviation from a straight line passing through the actual endpoints of the VFC transfer function. The error is expressed in % of the frequency span: Frequency Span = fOUT(max) – fOUT(min) The parts also contain an on-chip +2.5 V bandgap reference and operate from a single +5 V supply. A block diagram of the AD7742 is shown in Figure 2. OFFSET ERROR This is a measure of the offset error of the VFC. Ideally, the minimum output frequency (corresponding to minimum input voltage) is 5% of fCLKIN The deviation from this value is the offset error. It is expressed in terms of the error referred to the input voltage. It is expressed in mV. INTEGRATOR VIN1 VIN2 GAIN ERROR VIN3 This is a measure of the span error of the VFC. The gain is the scale factor that relates the input VIN to the output fOUT. The gain error is the deviation in slope of the actual VFC transfer characteristic from the ideal expressed as a percentage of the full-scale span. VIN4 fOUT INPUT MUX SWITCHED CAPS Figure 2. AD7742 Block Diagram Input Amplifier Stage The buffered input stage for the analog inputs presents a high impedance, allowing significant external source impedances. The four analog inputs (VIN1 through VIN4) each have a voltage range from +0.5 V to VDD – 1.75 V. This is an absolute voltage range and is relative to the GND pin. OFFSET ERROR DRIFT This is a measure of the change in Offset Error with changes in temperature. It is expressed in µV/°C. GAIN ERROR DRIFT In the case of the AD7742 multichannel part, a differential multiplexer switches one of the differential input channels to the VFC modulator. The multiplexer is controlled by two pins, A1 and A0. See Table I for channel configurations. This is a measure of the change in Gain Error with changes in temperature. It is expressed in (ppm of span)/°C. POWER-SUPPLY REJECTION RATIO (PSRR) This indicates how the output of the VFC is affected by changes in the supply voltage. Again, this error is referred to the input voltage. The input voltage is kept constant and the VDD supply is varied ± 5%. The ratio of the apparent change in input voltage to the change in VDD is measured in dBs. Table I. AD7742 Input Channel Selection CHANNEL-TO-CHANNEL ISOLATION This is a ratio of the amplitude of the signal at the input of one channel to a sine wave on the input of another channel. It is measured in dBs. A1 A0 VIN(+) VIN(–) Type 0 0 1 1 0 1 0 1 VIN1 VIN2 VIN3 VIN1 VIN4 VIN4 VIN4 VIN2 Pseudo Differential Pseudo Differential Full Differential Full Differential Analog Input Ranges The AD7741 has a unipolar single-ended input channel whereas the AD7742 contains four input channels which may be configured as two fully differential channels or as three pseudodifferential channels. The AD7742 also has a X1/X2 gain option on the front end. The channel and gain settings are pin-programmable. COMMON-MODE REJECTION For the AD7742, the output frequency should remain unchanged provided the differential input remains unchanged although its common-mode level may change. The CMR is the ratio of the apparent change in differential input voltage to the actual change in common-mode voltage. It is expressed in dBs. REV. 0 COMPARATOR SWITCHED CAPS The AD7742 uses differential inputs to provide common-mode noise rejection (i.e., the converted result will correspond to the differential voltage between the two inputs). The absolute voltage on both inputs must lie between +0.5 V and VDD –1.75 V. –7– AD7741/AD7742 Table II. AD7741/AD7742 Input Range Selection UNI/BIP GAIN Gain, G VIN(Min) fOUT = 0.05 fCLKIN VIN(Max) fOUT = 0.45 fCLKIN Part N/A 0 0 1 1 N/A 0 1 0 1 X1 X1 X2 X1 X2 0 –VREF –VREF/2 0 0 +VREF +VREF +VREF/2 +VREF +VREF/2 AD7741 AD7742 AD7742 AD7742 AD7742 As can be seen from Table II, the AD7741 has one input range configuration whereas the AD7742 has unipolar/bipolar as well as gain options depending on the status of the GAIN and UNI/BIP pins. OUTPUT FREQUENCY fOUT fOUTMAX (0.45 fCLKIN) The transfer function for the AD7741 is shown in Figure 3. Figure 4 shows the AD7742 transfer function for unipolar input range configuration while the AD7742 transfer function for bipolar input range configuration is shown in Figure 5. fOUTMIN (0.05 fCLKIN) OUTPUT FREQUENCY fOUT VREF – fOUTMAX (0.45 fCLKIN) + GAIN VREF DIFFERENTIAL INPUT VOLTAGE GAIN Figure 5. AD7742 Transfer Characteristic for Bipolar Differential Input Range: –VREF/Gain to +VREF/Gain; the common-mode range must be between +0.5 V and VDD – 1.75 V. UNI/BIP pin tied to GND. VFC Modulator fOUTMIN The analog input signal to the AD7741/AD7742 is continuously sampled by a switched capacitor modulator whose sampling rate is set by a master clock input that may be supplied externally or by a crystal-controlled on-chip clock oscillator. However, the input signal is buffered on-chip before being applied to the sampling capacitor of the modulator. This isolates the sampling capacitor charging currents from the analog input pins. (0.05 fCLKIN) 0 REFIN INPUT VOLTAGE VIN Figure 3. AD7741 Transfer Characteristic for Input Range from 0 to VREF OUTPUT FREQUENCY fOUT This system is a negative feedback loop that tries to keep the net charge on the integrator capacitor at zero, by balancing charge injected by the input voltage with charge injected by the VREF. The output of the comparator provides the digital input for the 1-bit DAC, so that the system functions as a negative feedback loop that tries to minimize the difference signal (see Figure 6). fOUTMAX (0.45 fCLKIN) CLK fOUTMIN (0.05 fCLKIN) INTEGRATOR COMPARATOR 0 + VREF GAIN DIFFERENTIAL INPUT VOLTAGE INPUT + S + 1-BIT STREAM – – Figure 4. AD7742 Transfer Characteristic for Unipolar Differential Input Range: 0 V to VREF/Gain; the input common-mode range must be between +0.5 V and VDD – 1.75 V. UNI/BIP pin tied to VDD. +VREF –VREF Figure 6. AD7741/AD7742 Modulator Loop –8– REV. 0 AD7741/AD7742 The digital data that represents the analog input voltage is contained in the duty cycle of the pulse train appearing at the output of the comparator. The output is a fixed-width pulse whose frequency depends on the analog input signal. The input voltage is offset internally so that a full-scale input gives an output frequency of 0.45 fCLKIN and zero-scale input gives an output frequency of 0.05 fCLKIN. The output allows simple interfacing to either standard logic families or opto-couplers. The clock high period controls the pulsewidth of the frequency output. The pulse is initiated by the edge of the clock signal. The delay time between the edge of the clock and the edge of the frequency output is typically 9 ns. Figure 7 shows the waveform of this frequency output. AD7741/AD7742 TO OTHER CIRCUITRY 5MV CLKIN C1 C2 Figure 8. On-Chip Oscillator The on-chip oscillator circuit also has a start-up time associated with it before it oscillates at its correct frequency and correct voltage levels. The typical start-up time for the circuit is 5 ms (with a 6.144 MHz crystal). After power-up, or if there is a step change in input voltage, there is a settling time that must elapse before valid data is obtained. This is typically 2 CLKIN cycles on the AD7742 and 10 CLKIN cycles on the AD7741. The AD7741/AD7742 master clock appears on the CLKOUT pin of the device. The maximum recommended load on this pin is one CMOS load. When using a crystal to generate the AD7741/ AD7742 clock it may be desirable to then use this clock as the clock source for the system. In this case it is recommended that the CLKOUT signal be buffered with a CMOS buffer before being applied to the rest of the circuit. fCLKIN fOUT = fCLKIN/4 VIN = VREF/2 fOUT = fCLKIN/10 VIN = VREF/8 Reference Input The AD7741/AD7742 performs conversion relative to an applied reference voltage that allows easy interfacing to ratiometric systems. This reference may be applied using the internal 2.5 V bandgap reference. For the AD7741, this is done by simply leaving REFIN/OUT unconnected. For the AD7742, REFIN is tied to REFOUT. Alternatively, an external reference, e.g., REF192 or AD780, may be used. For the AD7741, this is connected to REFIN/OUT and will overdrive the internal reference. For the AD7742, it is connected directly to the REFIN pin. fOUT = fCLKIN*3/20 VIN = VREF/4 6 TCLK 7 TCLK AVERAGE fOUT IS fCLKIN *3/20 BUT THE ACTUAL PULSE STREAM VARIES BETWEEN fCLKIN/6 AND fCLKIN/7 Figure 7. AD7741/AD7742 Frequency Output Waveforms Clock Generation As distinct from the asynchronous VFCs which rely on the stability of an external capacitor to set their full-scale frequency, the AD7741/AD7742 uses an external clock to define the full-scale output frequency. The result is a more stable, more linear transfer function and also allows the designer to determine the system stability and drift based upon the external clock selected. A crystal oscillator may also be used if desired. While the internal reference will be adequate for most applications, power supply rejection and overall regulation may be improved through the use of an external precision reference. The process of selecting an external voltage reference should include consideration of drive capability, initial error, noise and drift characteristics. A suitable choice would be the AD780 or REF192. The AD7741/AD7742 requires a master clock input, which may be an external CMOS-compatible clock signal applied to the CLKIN pin (CLKOUT not used). Alternatively, a crystal of the correct frequency can be connected between CLKIN and CLKOUT, when the clock circuit will function as a crystal controlled oscillator. Figure 8 shows a simple model of the onchip oscillator. REV. 0 CLKOUT Power-Down Mode The low power standby mode is initiated by taking the PD pin low, which shuts down most of the analog and digital circuitry. This reduces the power consumption to 185 µW max. –9– AD7741/AD7742 APPLICATIONS The basic connection diagram for the part is shown in Figure 9. In the connection diagram shown, the AD7742 analog inputs are configured as fully differential, bipolar inputs with a gain of 1. A quartz crystal provides the master clock source for the part. It may be necessary to connect capacitors (C1 and C2 in the diagram) on the crystal to ensure that it does not oscillate at overtones of its fundamental operating frequency. The values of capacitors will vary depending on the manufacturer’s specifications. VIN AD7741 fOUT COUNTER TO mP GATE SIGNAL CLKIN FREQUENCY DIVIDER CLOCK GENERATOR Figure 10. A/D Conversion Using the AD7741 VFC +5V 4096x TCLOCK fCLKIN VDD VIN1 DIFF INPUT 1 PD REFOUT VIN2 fOUT REFIN VIN3 DIFF INPUT 2 AD7742 VIN4 GATE GND A0 CHANNEL SELECT fOUT UNI/BIP A1 TGATE Figure 11. Waveforms in an A/D Converter Using a VFC GAIN CLKIN C1 The clock frequency and the gate time determine the resolution of such an ADC. If 12-bit resolution is required and fCLKIN is 5 MHz (therefore, fOUT max is 2.25 MHz), the minimum gate time required is calculated as follows: CLKOUT C2 N counts at Full Scale (2.25 MHz) will take Figure 9. Basic Connection Diagram (N/2.25 × 106) seconds = minimum gate time. A/D Conversion Techniques Using the AD7741/AD7742 When used as an ADC, VFCs provide certain advantages including accuracy, linearity and being inherently monotonic. The AD7741/AD7742 has a true integrating input which smooths out noise peaks. The most popular method of using a VFC in an A/D system is to count the output pulses of fOUT for a fixed gate interval (see Figure 10). This fixed gate interval should be generated by dividing down the clock input frequency. This ensures that any errors due to clock jitter or clock frequency drift are eliminated. The ratio of the fOUT to the clock frequency is what is important here, not the absolute value of fOUT. The frequency division can be done by a binary counter where fCLKIN is the CLK input. Figure 11 shows the waveforms of fCLKIN, fOUT and the Gate signal. A counter counts the rising edges of fOUT while the Gate signal is high. Since the gate interval is not synchronized with fOUT, there is a possibility of a counting inaccuracy. Depending on fOUT, an error of one count may occur. N is the total number of codes for a given resolution; 4096 for 12 bits minimum gate time = (4096/2.25 × 106) sec = 1.820 ms. Since TGATE × fOUT max = number of counts at full scale, a faster conversion with the same resolution can be performed with a higher fOUT max. This high fOUT max (3 MHz) is a main feature of the AD7741/AD7742. If the output frequency is measured by counting pulses gated to a signal which is derived from the clock, the clock stability is unimportant and the device simply performs as a voltagecontrolled frequency divider, producing a high resolution ADC. The inherent monotonicity of the transfer function and wide range of input clock frequencies allows the conversion time and resolution to be optimized for specific applications. There is another parameter is taken into account when choosing the length of the gate interval. Because the integration period of the system is equal to the gate interval, any interfering signal can be rejected by counting for an integer number of periods of the interfering signal. For example, a gate interval of 100 ms will give normal-mode rejection of 50 Hz and 60 Hz signals. –10– REV. 0 AD7741/AD7742 Isolation Applications Power Supply Bypassing and Grounding In addition to analog-to-digital conversion, the AD7741/AD7742 can be used in isolated analog signal transmission applications. Due to noise, safety requirements or distance, it may be necessary to isolate the AD7741/AD7742 from any controlling circuitry. This can easily be achieved by using opto-isolators, which will provide isolation in excess of 3 kV. In any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. The printed circuit board housing the AD7741/AD7742 should be designed so the analog and digital sections are separated and confined to certain areas of the board. Opto-electronic coupling is a popular method of isolated signal coupling. In this type of device, the signal is coupled from an input LED to an output photo-transistor, with light as the connecting medium. This technique allows dc to be transmitted, is extremely useful in overcoming ground loops between equipment, and is applicable over a wide range of speeds and power. The analog voltage to be transmitted is converted to a pulse train using the VFC. An opto-isolator circuit is used to couple this pulse train across an isolation barrier using light as the connecting medium. The input LED of the isolator is driven from the output of the AD7741/AD7742. At the receiver side, the output transistor is operated in the photo-transistor mode. The pulse train can be reconverted to an analog voltage using a frequency-to-voltage converter; alternatively, the pulse train can be fed into a counter to generate a digital signal. The analog and digital sections of the AD7741/AD7742 have been designed to allow operation from a single-ended power source, simplifying its use with isolated power supplies. Figure 12 shows a general purpose VFC circuit using a low cost opto-isolator. A +5 V power supply is assumed for both the isolated (+5 V isolated) and local (+5 V local) supplies. VCC +5V VDD R IN AD774x OPTOCOUPLER fOUT GND2 GND1 To minimize capacitive coupling between them, digital and analog ground planes should only be joined in one place, close to the DUT and should not overlap. Avoid running digital lines under the device as these will couple noise onto the die. The analog ground plane should be allowed to run under the AD7742 to avoid noise coupling. The power supply lines to the AD7742 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 parts of the board and clock signals should never be run near 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 effect of feedthrough through the board. A microstrip technique is by far the best but is not always possible with a double-sided board. In this technique, the component side of the board is dedicated to the ground plane while the signal traces are placed on the solder side. Good decoupling is also important. All analog supplies should be decoupled to GND with surface mount capacitors, 10 µF in parallel with 0.1 µF located as close to the package as possible, ideally right up against the device. The lead lengths on the bypass capacitor should be as short as possible. It is essential that these capacitors be placed physically close to the AD7741/AD7742 to minimize the inductance of the PCB trace between the capacitor and the supply pin. The 10 µF are the tantalum bead type and are located in the vicinity of the VFC to reduce lowfrequency ripple. The 0.1 µF capacitors should have low Effective Series Resistance (ESR) and Effective Series Inductance (ESI), such as the common ceramic types, which provide a low impedance path to ground at high frequencies to handle transient currents due to internal logic switching. Additionally, it is beneficial to have large capacitors (> 47 µF) located at the point where the power connects to the PCB. ISOLATION BARRIER Figure 12. Opto-Isolated Application REV. 0 –11– AD7741/AD7742 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 8-Lead SO (R-8) 0.430 (10.92) 0.348 (8.84) 8 5 1 PIN 1 0.1968 (5.00) 0.1890 (4.80) 0.280 (7.11) 0.240 (6.10) 0.1574 (4.00) 0.1497 (3.80) 4 0.100 (2.54) BSC 0.210 (5.33) MAX C3601–8–5/99 8-Lead Plastic DIP (N-8) 0.325 (8.25) 0.300 (7.62) 0.060 (1.52) 0.015 (0.38) 4 0.2440 (6.20) 0.2284 (5.80) 0.0196 (0.50) 3 458 0.0099 (0.25) 0.0500 (1.27) BSC 0.195 (4.95) 0.115 (2.93) 0.102 (2.59) 0.094 (2.39) 0.0098 (0.25) 0.0040 (0.10) SEATING PLANE 0.015 (0.381) 0.008 (0.204) 0.022 (0.558) 0.070 (1.77) SEATING 0.014 (0.356) 0.045 (1.15) PLANE 5 1 PIN 1 0.130 (3.30) MIN 0.160 (4.06) 0.115 (2.93) 8 16-Lead Plastic DIP (N-16) 0.0192 (0.49) 0.0138 (0.35) 0.0098 (0.25) 0.0075 (0.19) 88 08 0.0500 (1.27) 0.0160 (0.41) 16-Lead Narrow Body SO (R-16A) 0.840 (21.34) 0.745 (18.92) 0.3937 (10.00) 0.3859 (9.80) 9 1 8 0.280 (7.11) 0.240 (6.10) PIN 1 0.060 (1.52) 0.015 (0.38) 0.210 (5.33) MAX 0.130 (3.30) 0.160 (4.06) MIN 0.115 (2.93) 0.022 (0.558) 0.100 0.070 (1.77) SEATING PLANE (2.54) 0.014 (0.356) 0.045 (1.15) BSC 0.1574 (4.00) 0.1497 (3.80) 0.325 (8.25) 0.300 (7.62) PIN 1 0.195 (4.95) 0.115 (2.93) 0.015 (0.381) 0.008 (0.204) 16 9 1 8 0.050 (1.27) BSC 0.0098 (0.25) 0.0040 (0.10) 0.2440 (6.20) 0.2284 (5.80) 0.0688 (1.75) 0.0532 (1.35) 0.0196 (0.50) 3 458 0.0099 (0.25) 88 0.0192 (0.49) SEATING 0.0099 (0.25) 08 0.0500 (1.27) PLANE 0.0138 (0.35) 0.0160 (0.41) 0.0075 (0.19) PRINTED IN U.S.A. 16 –12– REV. 0