19-1066; Rev 1; 11/09 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC ____________________________Features The MAX1245 12-bit data-acquisition system combines an 8-channel multiplexer, high-bandwidth track/hold, and serial interface with high conversion speed and ultra-low power consumption. It operates from a single +2.375V to +3.3V supply, and its analog inputs are software configurable for unipolar/bipolar and single-ended/differential operation. ♦ Single +2.375V to +3.3V Operation ♦ 8-Channel Single-Ended or 4-Channel Differential Analog Inputs ♦ Low Power: 0.8mA (100ksps) 10µA (1ksps) 1µA (power-down mode) ♦ Internal Track/Hold, 100kHz Sampling Rate ♦ SPI/QSPI/MICROWIRE/TMS320-Compatible 4-Wire Serial Interface ♦ Software-Configurable Unipolar or Bipolar Inputs ♦ 20-Pin DIP/SSOP Packages The 4-wire serial interface directly connects to SPI™, QSPI™, and MICROWIRE™ devices without external logic. A serial strobe output allows direct connection to TMS320-family digital signal processors. The MAX1245 works with an external reference, and uses either the internal clock or an external serial-interface clock to perform successive-approximation analog-to-digital conversions. This device provides a hard-wired SHDN pin and a software-selectable power-down, and can be programmed to automatically shut down at the end of a conversion. Accessing the serial interface powers up the MAX1245, and the quick turn-on time allows it to be shut down between conversions. This technique can cut supply current to under 10µA at reduced sampling rates. The MAX1245 is available in a 20-pin DIP package and an SSOP that occupies 30% less area than an 8-pin DIP. For supply voltages from +2.7V to +5.25V, use the pincompatible MAX147. ________________________Applications Portable Data Logging Medical Instruments Battery-Powered Instruments Data Acquisition ___________Typical Operating Circuit +2.5V 0V to +2.048V ANALOG INPUTS VDD VDD CH0 0.1μF CPU MAX1245 +2.048V 0.1μF VREF CS SCLK DIN DOUT SSTRB SHDN PART* TEMP RANGE I/O SCK (SK)* MOSI (SO) MISO (SI) PIN-PACKAGE INL (LSB) ±1/2 MAX1245ACPP 0°C to +70°C 20 Plastic DIP MAX1245BCPP 0°C to +70°C 20 Plastic DIP MAX1245ACAP 0°C to +70°C 20 SSOP 20 SSOP ±1 ±1/2 ±1 MAX1245BCAP 0°C to +70°C MAX1245AEPP -40°C to +85°C 20 Plastic DIP ±1/2 MAX1245BEPP -40°C to +85°C 20 Plastic DIP ±1 MAX1245AEAP -40°C to +85°C 20 SSOP ±1/2 MAX1245BEAP -40°C to +85°C 20 SSOP ±1 *Contact factory for availability of alternate surface-mount packages. ___________________Pin Configuration TOP VIEW CH0 1 20 VDD CH1 2 19 SCLK CH2 3 18 CS CH3 4 DGND AGND COM CH7 ________________Ordering Information MAX1245 17 DIN CH4 5 16 SSTRB CH5 6 15 DOUT CH6 7 14 DGND CH7 8 13 AGND COM 9 12 VDD SHDN 10 11 VREF VSS DIP/SSOP SPI and QSPI are trademarks of Motorola, Inc. MICROWIRE is a trademark of National Semiconductor Corp. ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX1245 ________________General Description MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC ABSOLUTE MAXIMUM RATINGS SSOP (derate 8.00mW/°C above +70°C) ................... 640mW CERDIP (derate 11.11mW/°C above +70°C) .............. 889mW Operating Temperature Ranges MAX1245_C_P ................................................... 0°C to +70°C MAX1245_E_P ................................................ -40°C to +85°C Storage Temperature Range ............................ -60°C to +150°C Lead Temperature (soldering, 10sec) ............................ +300°C VDD to AGND, DGND .............................................. -0.3V to +6V AGND to DGND.................................................... -0.3V to +0.3V CH0–CH7, COM to AGND, DGND ............ -0.3V to (VDD + 0.3V) VREF to AGND........................................... -0.3V to (VDD + 0.3V) Digital Inputs to DGND............................................ -0.3V to +6V Digital Outputs to DGND ........................... -0.3V to (VDD + 0.3V) Digital Output Sink Current .................................................25mA Continuous Power Dissipation (TA = +70°C) Plastic DIP (derate 11.11mW/°C above +70°C) ......... 889mW Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. ELECTRICAL CHARACTERISTICS (VDD = +2.375V to +3.3V, COM = 0V, fCLK = 1.5MHz, external clock (50% duty cycle), 15 clocks/conversion cycle (100ksps), VREF = 2.048V applied to VREF pin, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS DC ACCURACY (Note 1) Resolution MIN TYP MAX 12 Relative Accuracy (Note 2) INL Differential Nonlinearity DNL UNITS Bits MAX1245A ±0.5 MAX1245B ±1.0 No missing codes over temperature LSB ±1 LSB Offset Error ±0.5 ±4 LSB Gain Error (Note 3) ±0.5 ±4 LSB Gain Temperature Coefficient ±0.25 ppm/°C Channel-to-Channel Offset Matching ±0.2 LSB DYNAMIC SPECIFICATIONS (10kHz sine-wave input, 0Vp-p to 2.048Vp-p, 100ksps, 1.5MHz external clock, bipolar input mode) Signal-to-Noise + Distortion Ratio SINAD 68 dB Total Harmonic Distortion THD Spurious-Free Dynamic Range SFDR Up to the 5th harmonic -76 76 dB dB Channel-to-Channel Crosstalk 50kHz, 2Vp-p (Note 4) -85 dB Small-Signal Bandwidth -3dB rolloff 2.25 MHz 1.0 MHz Full-Power Bandwidth CONVERSION RATE Conversion Time (Note 5) Track/Hold Acquisition Time tCONV tACQ Internal clock, SHDN = FLOAT 5.5 7.5 Internal clock, SHDN = VDD 35 65 µs External clock = 1.5MHz, 12 clocks/conversion 8 2.0 µs External clock = 1.5MHz Aperture Delay 40 ns Aperture Jitter <50 ps Internal Clock Frequency External Clock Frequency 2 SHDN = FLOAT 1.5 SHDN = VDD Data transfer only MHz 0.225 0.1 1.5 0 1.5 _______________________________________________________________________________________ MHz +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC (VDD = +2.375V to +3.3V, COM = 0V, fCLK = 1.5MHz, external clock (50% duty cycle), 15 clocks/conversion cycle (100ksps), VREF = 2.048V applied to VREF pin, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS ANALOG/COM INPUTS Input Voltage Range, SingleEnded and Differential (Note 6) Unipolar, COM = 0V Multiplexer Leakage Current On/off leakage current, VIN = 0V or VDD Input Capacitance (Note 7) 0 to VREF Bipolar, COM = VREF/2 ±VREF/2 ±0.01 ±1 16 V µA pF EXTERNAL REFERENCE VREF Input Voltage Range (Note 8) 1.0 VREF Input Current VREF = 2.048V VREF Input Resistance 82 18 0.01 DIN, SCLK, CS Input Low Voltage DIN, SCLK, CS Input Hysteresis IIN VIN = 0V or VDD DIN, SCLK, CS Input Capacitance CIN (Note 7) SHDN Input High Voltage VINH SHDN Input Low Voltage VINL VIM SHDN Voltage, Floating VFLT SHDN Maximum Allowed Leakage, Mid Input kΩ 10 ±0.01 V V ±1 µA 15 pF VDD - 0.4 V SHDN = 0V or VDD VDD/2 - 0.3 SHDN = open µA V 0.2 DIN, SCLK, CS Input Leakage SHDN Input Mid Voltage µA 0.8 VHYST IIN 120 2.0 VINL SHDN Input Current V 25 Shutdown VREF Input Current DIGITAL INPUTS (DIN, SCLK, CS, SHDN) VINH DIN, SCLK, CS Input High Voltage VDD + 50mV 0.4 V ±4.0 µA VDD/2 + 0.3 V VDD/2 SHDN = open V ±80 nA DIGITAL OUTPUTS (DOUT, SSTRB) Output Voltage Low VOL Output Voltage High VOH Three-State Leakage Current Three-State Output Capacitance POWER REQUIREMENTS Positive Supply Voltage Positive Supply Current Supply Rejection (Note 9) IL COUT ISINK = 5mA ISOURCE =0.5mA PSR 0.5 VDD - 0.375 CS = VDD V V ±0.01 CS = VDD (Note 7) VDD IDD 0.4 ISINK = 16mA 2.375 ±10 µA 15 pF 3.3 V Operating mode, full-scale input 0.8 1.3 mA Power-down 1.2 10 µA VDD = 2.375V to 3.3V, full-scale input, external reference = 2.048V ±0.3 mV _______________________________________________________________________________________ 3 MAX1245 ELECTRICAL CHARACTERISTICS (continued) TIMING CHARACTERISTICS (VDD = +2.375V to +3.3V, COM = 0V, TA = TMIN to TMAX, unless otherwise noted.) PARAMETER SYMBOL Acquisition Time CONDITIONS MIN tACQ 2.0 DIN to SCLK Setup tDS 200 DIN to SCLK Hold tDH SCLK Fall to Output Data Valid tDO Figure 1 CS Fall to Output Enable tDV Figure 1 tTR Figure 2 CS Rise to Output Disable TYP MAX UNITS µs ns 20 0 ns 260 ns 240 ns 400 ns CS to SCLK Rise Setup tCSS 200 ns CS to SCLK Rise Hold tCSH 0 ns SCLK Pulse Width High tCH 300 ns SCLK Pulse Width Low tCL 300 SCLK Fall to SSTRB tSSTRB ns Figure 1 260 ns tSDV External clock mode only, Figure 1 240 ns CS Rise to SSTRB Output Disable tSTR External clock mode only, Figure 2 400 ns SSTRB Rise to SCLK Rise tSCK Internal clock mode only (Note 7) CS Fall to SSTRB Output Enable Note 1: Note 2: Note 3: Note 4: Note 5: Note 6: Note 7: Note 8: Note 9: 0 ns Tested at VDD = +2.375V; COM = 0V; unipolar single-ended input mode. Relative accuracy is the deviation of the analog value at any code from its theoretical value after the full-scale range has been calibrated. External reference (VREF = +2.048V), offset nulled. Ground “on” channel; sine wave applied to all “off” channels. Conversion time defined as the number of clock cycles multiplied by the clock period; clock has 50% duty cycle. The common-mode range for the analog inputs is from AGND to VDD. Guaranteed by design. Not subject to production testing. ADC performance is limited by the converter’s noise floor, typically 300µVp-p. Measured as |VFS(2.375V) - VFS(3.3V)|. __________________________________________Typical Operating Characteristics (VDD = 2.5V, VREF = 2.048V, fCLK = 1.5MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.) RL = ∞ CODE = 101010100000 0.50 0.45 0.40 0.85 0.35 0.75 0.80 INL (LSB) CLOAD = 50pF IDD (mA) 1.00 MAX1245-03 RL = ∞ CODE = 101010100000 MAX1245-02 0.90 MAX1245-01 1.25 INTEGRAL NONLINEARITY vs. SUPPLY VOLTAGE SUPPLY CURRENT vs. TEMPERATURE SUPPLY CURRENT vs. SUPPLY VOLTAGE IDD (mA) MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC 0.75 CLOAD = 20pF 0.30 0.25 0.20 0.15 0.10 0.70 0.05 0.50 2.375 4 2.625 2.875 VDD (V) 3.125 3.375 0.65 -55 -30 -5 20 45 70 95 TEMPERATURE (°C) 120 145 0.00 2.375 2.625 2.875 VDD (V) _______________________________________________________________________________________ 3.125 3.375 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC INTEGRAL NONLINEARITY vs. TEMPERATURE 0.35 0.25 0.20 0.30 0.25 0.20 0.15 0.15 0.15 0.10 0.10 0.10 0.05 0.05 0.05 20 45 70 95 TEMPERATURE (˚C) 120 145 0.45 0.35 0.30 0.25 0.20 3.125 0 -55 3.375 0.40 0.35 0.35 0.30 0.25 0.20 0.25 0.20 0.10 0.05 0 -55 -30 -5 VDD (V) 0 2.375 120 145 0.45 0.30 0.25 0.20 0.15 0.40 0.35 0.30 0.25 0.20 0.15 0.35 0.30 0.25 0.20 0.15 0.10 0.10 0.05 0.05 0.05 -55 -30 -5 20 45 70 TEMPERATURE (˚C) 95 120 145 0 2.375 3.375 0.45 0.10 0 3.125 0.50 GAIN MATCHING (LSB) GAIN MATCHING (LSB) 0.35 2.875 CHANNEL-TO-CHANNEL GAIN MATCHING vs. TEMPERATURE 0.40 0.40 2.625 VDD (V) MAX1245-11 0.50 MAX1245-10 0.45 70 95 TEMPERATURE (˚C) 45 CHANNEL-TO-CHANNEL GAIN MATCHING vs. SUPPLY VOLTAGE GAIN ERROR vs. TEMPERATURE 0.50 20 120 145 0.30 0.05 3.375 95 0.45 0.40 0.05 3.125 70 0.50 0.15 2.875 45 GAIN ERROR vs. SUPPLY VOLTAGE 0.10 2.625 20 CHANNEL-TO-CHANNEL OFFSET MATCHING vs. TEMPERATURE 0.10 0 2.375 -5 TEMPERATURE (˚C) 0.15 0.15 -30 VDD (V) 0.45 OFFSET MATCHING (LSB) 0.40 2.875 0.50 MAX1245-07 0.50 2.625 GAIN ERROR (LSB) -5 MAX1245-08 -30 0 2.375 MAX1245-09 0.20 0.30 MAX1245-12 0.25 OFFSET (LSB) 0.40 0.35 CHANNEL-TO-CHANNEL OFFSET MATCHING vs. SUPPLY VOLTAGE GAIN ERROR (LSB) 0.45 0.35 0.30 MAX1245-06 0.45 0.40 0 -55 OFFSET MATCHING (LSB) MAX1245-05 VDD = 2.375V OFFSET vs. TEMPERATURE 0.50 0.40 OFFSET (LSB) INL (LSB) 0.45 OFFSET vs. SUPPLY VOLTAGE 0.50 MAX1245-04 0.50 0 2.625 2.875 VDD (V) 3.125 3.375 -55 -30 -5 20 45 70 95 TEMPERATURE (˚C) 120 145 _______________________________________________________________________________________ 5 MAX1245 ____________________________Typical Operating Characteristics (continued) (VDD = 2.5V, VREF = 2.048V, fCLK = 1.5MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.) ____________________________Typical Operating Characteristics (continued) (VDD = 2.5V, VREF = 2.048V, fCLK = 1.5MHz, CLOAD = 20pF, TA = +25°C, unless otherwise noted.) EFFECTIVE NUMBER OF BITS vs. INPUT FREQUENCY AVERAGE SUPPLY CURRENT vs. CONVERSION RATE EFFECTIVE NUMBER OF BITS VDD = VREF = 2.5V CODE = 101010100000 RL = ∞ IDD (μA) 100 10 8 CHANNELS 1 1 CHANNEL 0.1 MAX1245-14 12.0 MAX1245-13 1000 11.5 11.0 10.5 10.0 0.1 1 10 100 1k 10k 100k 1 10 100 INPUT FREQUENCY (kHz) CONVERSIONS PER CHANNEL PER SECOND (Hz) FFT PLOT INTEGRAL NONLINEARITY 20 0.25 0.20 fTONE = 10ksps fSAMPLE = 100ksps 0 0.15 -20 AMPLITUDE (dB) 0.10 INL (BITS) MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC 0.05 0 -0.05 -0.10 -40 -60 -80 -0.15 -100 -0.20 -120 -0.25 0 2048 DIGITAL CODE 6 4096 0 10 20 30 FREQUENCY (kHz) _______________________________________________________________________________________ 40 50 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC PIN NAME FUNCTION 1–8 CH0–CH7 9 COM Ground reference for analog inputs. Sets zero-code voltage in single-ended mode. Must be stable to ±0.5LSB. 10 SHDN Three-Level Shutdown Input. Pulling SHDN low shuts the MAX1245 down to 10µA (max) supply current; otherwise, the MAX1245 is fully operational. Letting SHDN float sets the internal clock frequency to 1.5MHz. Pulling SHDN high sets the internal clock frequency to 225kHz. See Hardware Power-Down section. 11 VREF External Reference Voltage Input for analog-to-digital conversion 12, 20 VDD Positive Supply Voltage 13 AGND Analog Ground 14 DGND Digital Ground 15 DOUT Serial Data Output. Data is clocked out at the falling edge of SCLK. High impedance when CS is high. 16 SSTRB Serial Strobe Output. In internal clock mode, SSTRB goes low when the MAX1245 begins the A/D conversion and goes high when the conversion is done. In external clock mode, SSTRB pulses high for one clock period before the MSB decision. High impedance when CS is high (external clock mode). 17 DIN Serial Data Input. Data is clocked in at the rising edge of SCLK. 18 CS Active-Low Chip Select. Data will not be clocked into DIN unless CS is low. When CS is high, DOUT is high impedance. 19 SCLK Sampling Analog Inputs Serial Clock Input. Clocks data in and out of serial interface. In external clock mode, SCLK also sets the conversion speed. (Duty cycle must be 40% to 60%.) VDD VDD DOUT DOUT CLOAD 50pF 6k CLOAD 50pF DGND DGND a) High-Z to VOH and VOL to VOH 6k b) High-Z to VOL and VOH to VOL Figure 1. Load Circuits for Enable Time 6k DOUT DOUT CLOAD 50pF 6k DGND a) VOH to High-Z CLOAD 50pF DGND b) VOL to High-Z Figure 2. Load Circuits for Disable Time _______________________________________________________________________________________ 7 MAX1245 ______________________________________________________________Pin Description MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC _______________Detailed Description The MAX1245 analog-to-digital converter (ADC) uses a successive-approximation conversion technique and input track/hold (T/H) circuitry to convert an analog signal to a 12-bit digital output. A flexible serial interface provides easy interface to microprocessors (µPs). No external hold capacitors are required. Figure 3 is a block diagram of the MAX1245. Pseudo-Differential Input The sampling architecture of the ADC’s analog comparator is illustrated in the equivalent input circuit (Figure 4). In single-ended mode, IN+ is internally switched to CH0–CH7, and IN- is switched to COM. In differential mode, IN+ and IN- are selected from the following pairs: CH0/CH1, CH2/CH3, CH4/CH5, and CH6/CH7. Configure the channels with Tables 2 and 3. In differential mode, IN- and IN+ are internally switched to either of the analog inputs. This configuration is pseudo-differential to the effect that only the signal at IN+ is sampled. The return side (IN-) must remain stable within ±0.5LSB (±0.1LSB for best results) with respect to AGND during a conversion. To accomplish this, connect a 0.1µF capacitor from IN- (the selected analog input) to AGND. During the acquisition interval, the channel selected as the positive input (IN+) charges capacitor CHOLD. The acquisition interval spans three SCLK cycles and ends on the falling SCLK edge after the last bit of the input control word has been entered. At the end of the acquisition interval, the T/H switch opens, retaining charge on CHOLD as a sample of the signal at IN+. CS SCLK 18 19 DIN 17 SHDN 10 CH0 CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM 1 VREF 11 2 3 4 5 6 Track/Hold The T/H enters its tracking mode on the falling clock edge after the fifth bit of the 8-bit control word has been shifted in. It enters its hold mode on the falling clock edge after the eighth bit of the control word has been shifted in. If the converter is set up for single-ended inputs, IN- is connected to COM, and the converter samples the “+” input. If the converter is set up for differential inputs, IN- connects to the “-” input, and the difference of |IN+ - IN-| is sampled. At the end of the conversion, the positive input connects back to IN+, and CHOLD charges to the input signal. The time required for the T/H to acquire an input signal is a function of how quickly its input capacitance is charged. If the input signal’s source impedance is high, the acquisition time lengthens, and more time must be allowed between conversions. The acquisition time, tACQ, is the maximum time the device takes to acquire the signal, and is also the minimum time needed for the signal to be acquired. It is calculated by: tACQ = 9 x (RS + RIN) x 16pF 12-BIT CAPACITIVE DAC VREF INPUT SHIFT REGISTER INT CLOCK CONTROL LOGIC CH0 CH1 OUTPUT SHIFT REGISTER ANALOG INPUT MUX T/H CLOCK IN 12-BIT SAR ADC 7 8 9 MAX1245 Figure 3. Block Diagram 8 The conversion interval begins with the input multiplexer switching CHOLD from the positive input, IN+, to the negative input, IN- (In single-ended mode, IN- is simply COM). This unbalances node ZERO at the input of the comparator. The capacitive DAC adjusts during the remainder of the conversion cycle to restore node ZERO to 0V within the limits of 12-bit resolution. This action is equivalent to transferring a charge of 16pF x [(VIN+) (VIN-)] from CHOLD to the binary-weighted capacitive DAC, which in turn forms a digital representation of the analog input signal. REF 15 16 SSTRB CH4 CH5 12, 20 14 OUT CH3 13 VDD DGND AGND CH6 CH7 ZERO 16pF CH2 DOUT COMPARATOR INPUT CHOLD MUX – + RIN 12k CSWITCH TRACK T/H SWITCH COM HOLD AT THE SAMPLING INSTANT, THE MUX INPUT SWITCHES FROM THE SELECTED IN+ CHANNEL TO THE SELECTED IN- CHANNEL. SINGLE-ENDED MODE: IN+ = CHO–CH7, IN- = COM. DIFFERENTIAL MODE: IN+ AND IN- SELECTED FROM PAIRS OF CH0/CH1, CH2/CH3, CH4/CH5, AND CH6/CH7. Figure 4. Equivalent Input Circuit _______________________________________________________________________________________ +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC Input Bandwidth The ADC’s input tracking circuitry has a 2.25MHz small-signal bandwidth, so it is possible to digitize high-speed transient events and measure periodic signals with bandwidths exceeding the ADC’s sampling rate by using undersampling techniques. To avoid high-frequency signals being aliased into the frequency band of interest, anti-alias filtering is recommended. Analog Input Protection Internal protection diodes, which clamp the analog input to VDD and AGND, allow the channel input pins to swing from AGND - 0.3V to VDD + 0.3V without damage. However, for accurate conversions near full scale, the inputs must not exceed VDD by more than 50mV or be lower than AGND by 50mV. If the analog input exceeds 50mV beyond the supplies, do not forward bias the protection diodes of off channels over two milliamperes, as excessive current will degrade the conversion accuracy of the on channel. Quick Look To quickly evaluate the MAX1245’s analog performance, use the circuit of Figure 5. The MAX1245 requires a control byte to be written to DIN before each conversion. Tying DIN to VDD feeds in control bytes of $FF (HEX), which trigger single-ended unipolar conversions on CH7 in external clock mode without powering down between conversions. In external clock mode, the SSTRB output pulses high for one clock period before the most significant bit of the 12-bit conversion result is shifted out of DOUT. Varying the analog input to CH7 alters the sequence of bits from DOUT. A total of 15 clock cycles is required per conversion. All transitions of the SSTRB and DOUT outputs occur on the falling edge of SCLK. How to Start a Conversion A conversion is started by clocking a control byte into DIN. With CS low, each rising edge on SCLK clocks a bit from DIN into the MAX1245’s internal shift register. After CS falls, the first arriving logic “1” bit defines the MSB of the control byte. Until this first “start” bit arrives, any number of logic “0” bits can be clocked into DIN with no effect. Table 1 shows the control-byte format. The MAX1245 is compatible with Microwire, SPI, and QSPI devices. For SPI, select the correct clock polarity and sampling edge in the SPI control registers: set CPOL = 0 and CPHA = 0. Microwire, SPI, and QSPI all transmit a byte and receive a byte at the same time. Using the Typical Operating Circuit, the simplest software interface requires only three 8-bit transfers to OSCILLOSCOPE VDD +2.5V 0.1μF DGND MAX1245 SCLK AGND COM 0V TO 2.048V ANALOG INPUT 0.01μF CS CH7 SSTRB SCLK +2.5V DIN DOUT 2.048V VREF SSTRB SHDN C1 0.1μF N.C. DOUT* 1.5MHz OSCILLATOR CH1 CH2 CH3 CH4 * FULL-SCALE ANALOG INPUT, CONVERSION RESULT = $FFF (HEX) Figure 5. Quick-Look Circuit _______________________________________________________________________________________ 9 MAX1245 where RIN = 12kΩ, RS = the source impedance of the input signal, and tACQ is never less than 2.0µs. Note that source impedances below 1kΩ do not significantly affect the AC performance of the ADC. Higher source impedances can be used if an input capacitor is connected to the analog inputs, as shown in Figure 5. Note that the input capacitor forms an RC filter with the input source impedance, limiting the ADC’s signal bandwidth. MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC Table 1. Control-Byte Format BIT 7 (MSB) BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0 (LSB) START SEL2 SEL1 SEL0 UNI/BIP SGL/DIF PD1 PD0 NAME DESCRIPTION BIT 7(MSB) START The first logic “1” bit after CS goes low defines the beginning of the control byte. 6 5 4 SEL2 SEL1 SEL0 These three bits select which of the eight channels are used for the conversion (Tables 2 and 3). 3 UNI/BIP 1 = unipolar, 0 = bipolar. Selects unipolar or bipolar conversion mode. In unipolar mode, an analog input signal from 0V to VREF can be converted; in bipolar mode, the signal can range from -VREF/2 to +VREF/2. 2 SGL/DIF 1 = single ended, 0 = differential. Selects single-ended or differential conversions. In singleended mode, input signal voltages are referred to COM. In differential mode, the voltage difference between two channels is measured (Tables 2 and 3). 1 0(LSB) PD1 PD0 Selects clock and power-down modes. PD1 PD0 Mode 0 0 Power-down (IQ = 1.2µA) 0 1 Unassigned 1 0 Internal clock mode 1 1 External clock mode DIF = 1) Table 2. Channel Selection in Single-Ended Mode (SGL/D SEL2 0 SEL1 0 SEL0 0 1 0 0 0 0 1 1 0 1 0 1 0 1 1 0 0 1 1 1 1 1 CH0 + CH1 CH2 CH3 CH4 CH5 CH6 CH7 COM – + – + – + – + – + – + – + – DIF = 0) Table 3. Channel Selection in Differential Mode (SGL/D SEL2 SEL1 SEL0 CH0 CH1 0 0 0 + – 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 10 – CH2 CH3 + – CH4 CH5 + – CH6 CH7 + – – + + – + – + ______________________________________________________________________________________ +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC Simple Software Interface Make sure the CPU’s serial interface runs in master mode so the CPU generates the serial clock. Choose a clock frequency from 100kHz to 1.5MHz. 1) Set up the control byte for external clock mode and call it TB1. TB1 should be of the format: 1XXXXX11 binary, where the Xs denote the particular channel and conversion mode selected. Clock Modes The MAX1245 may use either an external serial clock or the internal clock to perform the successive-approximation conversion. In both clock modes, the external clock shifts data in and out of the MAX1245. The T/H acquires the input signal as the last three bits of the control byte are clocked into DIN. Bits PD1 and PD0 of the control byte program the clock mode. Figures 7–10 show the timing characteristics common to both modes. 2) Use a general-purpose I/O line on the CPU to pull CS low. 3) Transmit TB1 and, simultaneously, receive a byte and call it RB1. Ignore RB1. 4) Transmit a byte of all zeros ($00 HEX) and, simultaneously, receive byte RB2. 5) Transmit a byte of all zeros ($00 HEX) and, simultaneously, receive byte RB3. External Clock In external clock mode, the external clock not only shifts data in and out, it also drives the analog-to-digital conversion. SSTRB pulses high for one clock period after the control byte’s last bit. Successive-approximation bit decisions are made and appear at DOUT on each of the next 12 SCLK falling edges (Figure 6). SSTRB and DOUT go into a high-impedance state when CS goes high; after the next CS falling edge, SSTRB outputs a logic low. Figure 8 shows the SSTRB timing in external clock mode. 6) Pull CS high. Figure 6 shows the timing for this sequence. Bytes RB2 and RB3 will contain the result of the conversion padded with one leading zero and three trailing zeros. The total conversion time is a function of the serial clock frequency and the amount of idle time between 8-bit transfers. Make sure that the total conversion time does not exceed 120µs, to avoid excessive T/H droop. The conversion must complete in some minimum time, or droop on the sample-and-hold capacitors may degrade conversion results. Use internal clock mode if the serial clock frequency is less than 100kHz, or if serial-clock interruptions could cause the conversion interval to exceed 120µs. CS tACQ SCLK 1 4 SEL2 SEL1 SEL0 UNI/ BIP DIN 8 SGL/ PD1 DIF 12 16 20 24 PD0 START SSTRB A/D STATE B11 MSB IDLE RB3 RB2 RB1 DOUT ACQUISITION 2.0μs B10 B9 B8 B7 B6 CONVERSION B5 B4 B3 B2 B1 B0 LSB FILLED WITH ZEROS IDLE (SCLK = 1.5MHz) Figure 6. 24-Clock External-Clock-Mode Conversion Timing (Microwire and SPI Compatible, QSPI Compatible with fCLK ≤ 1.5MHz) ______________________________________________________________________________________ 11 MAX1245 Digital Output In unipolar input mode, the output is straight binary (Figure 14). For bipolar inputs, the output is two’s-complement (Figure 15). Data is clocked out at the falling edge of SCLK in MSB-first format. perform a conversion (one 8-bit transfer to configure the ADC, and two more 8-bit transfers to clock out the 12-bit conversion result). See Figure 17 for MAX1245 QSPI connections. MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC ••• CS tCSH tCSS tCL tCH SCLK tCSH ••• tDS tDH ••• DIN tDV tDO tTR ••• DOUT Figure 7. Detailed Serial-Interface Timing ••• CS ••• tSTR tSDV SSTRB ••• ••• tSSTRB SCLK •••• tSSTRB ••• • PD0 CLOCKED IN Figure 8. External-Clock-Mode SSTRB Detailed Timing Internal Clock In internal clock mode, the MAX1245 generates its own conversion clock internally. This frees the µP from the burden of running the SAR conversion clock and allows the conversion results to be read back at the processor’s convenience, at any clock rate from zero to 1.5MHz. SSTRB goes low at the start of the conversion and then goes high when the conversion is complete. SSTRB will be low for a maximum of 7.5µs (SHDN = FLOAT), during which time SCLK should remain low for best noise performance. 12 An internal register stores data when the conversion is in progress. SCLK clocks the data out of this register at any time after the conversion is complete. After SSTRB goes high, the next falling clock edge produces the MSB of the conversion at DOUT, followed by the remaining bits in MSB-first format (Figure 9). CS does not need to be held low once a conversion is started. Pulling CS high prevents data from being clocked into the MAX1245 and three-states DOUT, but it does not adversely affect an internal clock-mode conversion already in progress. When internal clock mode is ______________________________________________________________________________________ +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC MAX1245 CS SCLK 1 2 3 4 5 SEL2 SEL1 SEL0 UNI/ BIP DIN 7 8 SGL/ PD1 DIF PD0 6 9 10 11 12 18 19 20 21 22 23 24 START SSTRB tCONV B11 MSB DOUT A/D STATE CONVERSION 7.5μs MAX (SCLK = 1.5MHz) (SHDN = FLOAT) ACQUISITION 2.0μs IDLE B10 B9 B2 B1 B0 LSB FILLED WITH ZEROS IDLE Figure 9. Internal Clock Mode Timing CS • • • tCONV tSCK tCSH tCSS SSTRB • • • tSSTRB SCLK • • • tDO PD0 CLOCK IN DOUT • • • NOTE: FOR BEST NOISE PERFORMANCE, KEEP SCLK LOW DURING CONVERSION. Figure 10. Internal Clock Mode SSTRB Detailed Timing selected, SSTRB does not go into a high-impedance state when CS goes high. Figure 10 shows the SSTRB timing in internal clock mode. In this mode, data can be shifted in and out of the MAX1245 at clock rates exceeding 1.5MHz, provided that the minimum acquisition time, tACQ, is kept above 2.0µs. SCLK, after the eighth bit of the control byte (the PD0 bit) is clocked into DIN. The start bit is defined as: The first high bit clocked into DIN with CS low any time the converter is idle; e.g., after VDD is applied. Data Framing If CS is toggled before the current conversion is complete, then the next high bit clocked into DIN is recognized as a start bit; the current conversion is terminated, and a new one is started. The falling edge of CS does not start a conversion on the MAX1245. The first logic high clocked into DIN is interpreted as a start bit and defines the first bit of the control byte. A conversion starts on the falling edge of OR The first high bit clocked into DIN after bit 5 of a conversion in progress is clocked onto the DOUT pin. ______________________________________________________________________________________ 13 MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC The fastest the MAX1245 can run is 15 clocks per conversion with CS held low between conversions. Figure 11a shows the serial-interface timing necessary to perform a conversion every 15 SCLK cycles in external clock mode. If CS is low and SCLK is continuous, guarantee a start bit by first clocking in 16 zeros. Most microcontrollers require that conversions occur in multiples of eight SCLK clocks; 16 clocks per conversion will typically be the fastest that a microcontroller can drive the MAX1245. Figure 11b shows the serial-interface timing necessary to perform a conversion every 16 SCLK cycles in external clock mode. DIN will be interpreted as a start bit. Until a conversion takes place, DOUT shifts out zeros. Power-Down The MAX1245’s automatic power-down mode can save considerable power when operating at speeds below the maximum sampling rate. Figure 13 shows the average supply current as a function of the sampling rate. You can save power by placing the converter in a lowcurrent shutdown state between conversions. Select power-down via bits 1 and 0 of the DIN control byte with SHDN high (Tables 1 and 4). Pull SHDN low at any time to shut down the converter completely. SHDN overrides bits 1 and 0 of the control byte (Table 5). Power-down mode turns off all chip functions that draw quiescent current, reducing IDD typically to 1.2µA. Figures 12a and 12b illustrate the various power-down sequences in both external and internal clock modes. __________ Applications Information Power-On Reset When power is first applied, and if SHDN is not pulled low, internal power-on reset circuitry activates the MAX1245 in internal clock mode, ready to convert with SSTRB = high. After the power supplies have stabilized, the internal reset time is 10µs, and no conversions should be performed during this phase. SSTRB is high on power-up and, if CS is low, the first logical 1 on Software Power-Down Software power-down is activated using bits PD1 and PD0 of the control byte. As shown in Table 4, PD1 and PD0 CS 1 8 1 8 1 SCLK S DIN S CONTROL BYTE 0 CONTROL BYTE 1 CONTROL BYTE 2 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 DOUT S CONVERSION RESULT 1 CONVERSION RESULT 0 SSTRB Figure 11a. External Clock Mode, 15 Clocks/Conversion Timing CS ••• SCLK ••• DIN DOUT S S CONTROL BYTE 0 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 CONVERSION RESULT 0 ••• CONTROL BYTE 1 B11 B10 B9 B8 CONVERSION RESULT 1 Figure 11b. External Clock Mode, 16 Clocks/Conversion Timing 14 ______________________________________________________________________________________ ••• +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC MAX1245 CLOCK MODE EXTERNAL EXTERNAL SHDN SETS SOFTWARE POWER-DOWN SETS EXTERNAL CLOCK MODE DIN S X X X X X 1 1 DOUT SETS EXTERNAL CLOCK MODE S X X X X X 0 0 S X X X X X 1 1 VALID DATA 12 DATA BITS 12 DATA BITS HARDWARE POWERDOWN POWERED UP POWERED UP MODE INVALID DATA SOFTWARE POWER-DOWN POWERED UP Figure 12a. Timing Diagram Power-Down Modes, External Clock CLOCK MODE DIN INTERNAL SETS POWER-DOWN SETS INTERNAL CLOCK MODE S X X X X X 1 0 S X X X X X 0 0 DOUT S DATA VALID DATA VALID CONVERSION SSTRB CONVERSION SOFTWARE POWER-DOWN POWERED UP MODE POWERED UP Figure 12b. Timing Diagram Power-Down Modes, Internal Clock Table 4. Software Power-Down and Clock Mode Table 5. Hard-Wired Power-Down and Internal Clock Frequency External Clock SHDN STATE DEVICE MODE INTERNAL CLOCK FREQUENCY Internal Clock 1 Enabled 225kHz 1 Unassigned Floating Enabled 1.5MHz 0 Power-Down 0 Power-Down N/A PD1 PD0 DEVICE MODE 1 1 1 0 0 0 ______________________________________________________________________________________ 15 Table 6. Full Scale and Zero Scale UNIPOLAR MODE BIPOLAR MODE Zero Scale VREF + COM COM also specify the clock mode. When software shutdown is asserted, the ADC continues to operate in the last specified clock mode until the conversion is complete. Then the ADC powers down into a low quiescent-current state. In internal clock mode, the interface remains active and conversion results can be clocked out after the MAX1245 has entered a software power-down. The first logical 1 on DIN is interpreted as a start bit, and powers up the MAX1245. Following the start bit, the data input word or control byte also determines clock mode and power-down states. For example, if the DIN word contains PD1 = 1, the chip remains powered up. If PD0 = PD1 = 0, a power-down resumes after one conversion. Hardware Power-Down Pulling SHDN low places the converter in hardware power-down. Unlike the software power-down mode, the conversion is not completed; it stops coincidentally with SHDN being brought low. SHDN also controls the clock frequency in internal clock mode. Letting SHDN float sets the internal clock frequency to 1.5MHz. When returning to normal operation with SHDN floating, there is a tRC delay of approximately 2MΩ x C L , where C L is the capacitive loading on the SHDN pin. Pulling SHDN high sets the internal clock frequency to 225kHz. This feature eases the settling-time requirement for the reference voltage. External Reference An external reference is required for the MAX1245. The reference voltage range is 1V to VDD. At VREF, the input impedance is a minimum of 18kΩ for DC currents. During a conversion, the reference must be able to deliver up to 250µA DC load current and have an output impedance of 10Ω or less. If the reference has higher output impedance or is noisy, bypass it close to the VREF pin with a 0.1µF capacitor. Transfer Function Table 6 shows the full-scale voltage ranges for unipolar and bipolar modes using a 2.048V reference. The external reference must have a temperature coefficient of 4ppm/°C or less to achieve accuracy to within 1LSB over the commercial temperature range of 0°C to +70°C. 16 Positive Full Scale VREF/2 + COM Zero Scale Negative Full Scale -VREF/2 + COM COM AVERAGE SUPPLY CURRENT vs. CONVERSION RATE 1000 MAX1245-13 Full Scale VDD = VREF = 2.5V CODE = 101010100000 RL = ∞ 100 IDD (μA) MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC 10 8 CHANNELS 1 1 CHANNEL 0.1 0.1 1 10 100 1k 10k 100k CONVERSIONS PER CHANNEL PER SECOND (Hz) Figure 13. Average Supply Current vs. Conversion Rate Figure 14 depicts the nominal, unipolar input/output (I/O) transfer function, and Figure 15 shows the bipolar input/output transfer function. Code transitions occur halfway between successive-integer LSB values. Output coding is binary, with 1LSB = 500µV (2.048V / 4096) for unipolar operation and 1LSB = 500µV [(2.048V / 2 - -2.048V / 2) / 4096] for bipolar operation. Layout, Grounding, and Bypassing For best performance, use printed circuit boards. Wire-wrap boards are not recommended. Board layout should ensure that digital and analog signal lines are separated from each other. Do not run analog and digital (especially clock) lines parallel to one another, or digital lines underneath the ADC package. Figure 16 shows the recommended system ground connections. A single-point analog ground (“star” ground point) should be established at AGND, separate from the logic ground. Connect all other analog grounds and DGND to the star ground. No other digital system ground should be connected to this ground. The ground return to the power supply for the star ______________________________________________________________________________________ +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC MAX1245 OUTPUT CODE OUTPUT CODE FULL-SCALE TRANSITION 11 . . . 111 FS = 011 . . . 110 ZS = COM 11 . . . 110 11 . . . 101 -VREF + COM 2 VREF 1LSB = 4096 -FS = 000 . . . 010 000 . . . 001 FS = VREF + COM ZS = COM VREF 1LSB = 4096 00 . . . 011 00 . . . 010 VREF + COM 2 011 . . . 111 000 . . . 000 111 . . . 111 111 . . . 110 111 . . . 101 100 . . . 001 00 . . . 001 100 . . . 000 00 . . . 000 0 (COM) 1 2 3 FS COM (≤VREF/2) - FS FS - 3/2LSB INPUT VOLTAGE (LSBs) INPUT VOLTAGE (LSBs) Figure 14. Unipolar Transfer Function, Full Scale (FS) = VREF + COM, Zero Scale (ZS) = COM ground should be low impedance and as short as possible for noise-free operation. High-frequency noise in the VDD power supply may affect the high-speed comparator in the ADC. Bypass the supply to the star ground with 0.1µF and 4.7µF capacitors close to pin 20 of the MAX1245. Minimize capacitor lead lengths for best supply-noise rejection. If the +2.5V power supply is very noisy, a 10Ω resistor can be connected as a lowpass filter (Figure 16). +FS - 1LSB Figure 15. Bipolar Transfer Function, Full Scale (FS) = VREF / 2 + COM, Zero Scale (ZS) = COM SUPPLIES +2.5V +2.5V GND +2.5V DGND R* = 10Ω VDD AGND COM DGND MAX1245 DIGITAL CIRCUITRY * OPTIONAL Figure 16. Power-Supply Grounding Connection ______________________________________________________________________________________ 17 MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC +2.5V +3V 0.1μF ANALOG INPUTS 4.7μF (POWER SUPPLIES) 1 CH0 VDD 20 2 CH1 SCLK 19 3 CH2 CS 18 PCS0 4 CH3 MAX1245 DIN 17 MOSI 5 CH4 SSTRB 16 6 CH5 DOUT 15 7 CH6 DGND 14 8 CH7 AGND 13 9 COM VDD 12 10 SHDN VREF 11 SCK MC683XX MISO (GND) 0.1μF +2.048V CLOCK CONNECTIONS NOT SHOWN Figure 17. MAX1245 QSPI Connections High-Speed Digital Interfacing with QSPI The MAX1245 can interface with QSPI using the circuit in Figure 17 (fSCLK = 1.5MHz, CPOL = 0, CPHA = 0). This QSPI circuit can be programmed to do a conversion on each of the eight channels. The result is stored in memory without taxing the CPU, since QSPI incorporates its own micro-sequencer. Because the maximum external clock frequency is 1.5MHz, the MAX1245 is QSPI compatible up to 1.5MHz. 18 ______________________________________________________________________________________ +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC CS XF SCLK CLKX 1) The TMS320 should be configured with CLKX (transmit clock) as an active-high output clock and CLKR (TMS320 receive clock) as an active-high input clock. CLKX and CLKR on the TMS320 are tied together with the MAX1245’s SCLK input. 2) The MAX1245’s CS pin is driven low by the TMS320’s XF_ I/O port, to enable data to be clocked into the MAX1245’s DIN. 3) An 8-bit word (1XXXXX11) should be written to the MAX1245 to initiate a conversion and place the device into external clock mode. Refer to Table 1 to select the proper XXXXX bit values for your specific application. 4) The MAX1245’s SSTRB output is monitored via the TMS320’s FSR input. A falling edge on the SSTRB output indicates that the conversion is in progress and data is ready to be received from the MAX1245. 5) The TMS320 reads in one data bit on each of the next 16 rising edges of SCLK. These data bits represent the 12-bit conversion result followed by four trailing bits, which should be ignored. MAX1245 TMS320LC3x-to-MAX1245 Interface Figure 18 shows an application circuit to interface the MAX1245 to the TMS320 in external clock mode. The timing diagram for this interface circuit is shown in Figure 19. Use the following steps to initiate a conversion in the MAX1245 and to read the results: TMS320LC3x MAX1245 CLKR DX DIN DR DOUT FSR SSTRB Figure 18. MAX1245-to-TMS320 Serial Interface 6) Pull CS high to disable the MAX1245 until the next conversion is initiated. CS SCLK DIN START SEL2 SEL1 SEL0 UNI/BIP SGL/DIF PD1 PD0 HIGH IMPEDANCE SSTRB DOUT MSB B10 B1 LSB HIGH IMPEDANCE Figure 19. TMS320 Serial-Interface Timing Diagram ______________________________________________________________________________________ 19 MAX1245 +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC Chip Information TRANSISTOR COUNT: 2554 20 Package Information For the latest package outline information and land patterns, go to www.maxim-ic.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE DOCUMENT NO. 20 PDIP A20-1 21-0056 20 SSOP P20-4 21-0043 ______________________________________________________________________________________ +2.375V, Low-Power, 8-Channel, Serial 12-Bit ADC REVISION NUMBER REVISION DATE 0 6/96 1 11/09 DESCRIPTION PAGES CHANGED Initial release. — Removed the dice package from the Ordering Information table. 1 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 ____________________ 21 © 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc. MAX1245 Revision History