MAX192 Low-Power, 8-Channel, Serial 10-Bit ADC - Robotics UWA

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Low-Power, 8-Channel,Serial 10-Bit ADCMAX19211 . . . 11111 . . . 11011 . . . 101OUTPUT CODEFULL-SCALETRANSITION011 . . . 111011 . . . 110000 . . . 010000 . . . 001000 . . . 000111 . . . 111111 . . . 110111 . . . 101FS = +4.09621LSB = +4.0961024FS = +4.096V1LSB = FS1024100 . . . 001100 . . . 00000 . . . 01100 . . . 01000 . . . 00100 . . . 0000 1 2 3FSINPUT VOLTAGE (LSBs)FS - 3/2LSB-FS0V+FS - 1LSBDIFFERENTIAL INPUT VOLTAGE (LSBs)Figure 15. Unipolar Transfer Function, 4.096V = Full Scale Figure 16. Differential Bipolar Transfer Function,±4.096V / 2 = Full Scaledummy conversion. This circuit combines fastmulti-channel conversion with lowest power consumptionpossible. Full power-down mode may provideincreased power savings in applications where theMAX192 is inactive for long periods of time, but whereintermittent bursts of high-speed conversions arerequired.External and Internal ReferencesThe MAX192 can be used with an internal or externalreference. Diode D1 shown in the Typical OperatingCircuit ensures correct start-up. Any standard signaldiode can be used. An external reference can eitherbe connected directly at the VREF terminal or at theREFADJ pin.The MAX192’s internally trimmed 2.46V reference isbuffered with a gain of 1.678 to scale an external 2.5Vreference at REFADJ to 4.096V at VREF.Internal ReferenceThe full-scale range of the MAX192 with internal referenceis 4.096V with unipolar inputs, and ±2.048V with differentialbipolar inputs. The internal reference voltage isadjustable to ±1.5% with the Reference-Adjust Circuit ofFigure 17.External ReferenceAn external reference can be placed at either theinput (REFADJ) or the output (VREF) of the internalbuffer amplifier. The REFADJ input impedance istypically 20kΩ. At VREF, the input impedance is aminimum of 12kΩ for DC currents. During conversion,an external reference at VREF must be able to deliverup to 350µA DC load current and have an outputimpedance of 10Ω or less. If the reference has higheroutput impedance or is noisy, bypass it close to theVREF pin with a 4.7µF capacitor.Using the buffered REFADJ input avoids externalbuffering of the reference. To use the direct VREF input,disable the internal buffer by tying REFADJ to V DD .Transfer Function and Gain AdjustFigure 15 depicts the nominal, unipolar input/output(I/O) transfer function, and Figure 16 shows the differentialbipolar input/output transfer function. Codetransitions occur halfway between successive integerLSB values. Output coding is binary with1LSB = 4.00mV (4.096V / 1024) for unipolar operationand 1LSB = 4.00mV [(4.096V / 2 - -4.096V / 2)/1024]for bipolar operation.18 ______________________________________________________________________________________
Low-Power, 8-Channel,Serial 10-Bit ADC100k+5V510k12MAX192REFADJ+5V GNDR* = 10ΩSUPPLIESMAX19224k0.01µFV DDAGNDDGND+5VDGNDMAX192DIGITALCIRCUITRY* OPTIONALFigure 17.Reference-Adjust CircuitFigure 18. Power-Supply Grounding ConnectionFigure 17, the Reference-Adjust Circuit, shows how toadjust the ADC gain in applications that use the internalreference. The circuit provides ±1.5% (±15LSBs) ofgain adjustment range.Layout, Grounding, BypassingFor best performance, use printed circuit boards.Wire-wrap boards are not recommended. Board layoutshould ensure that digital and analog signal lines areseparated from each other. Do not run analog and digital(especially clock) lines parallel to one another, ordigital lines underneath the ADC package.Figure 18 shows the recommended system groundconnections. A single-point analog ground (“star”ground point) should be established at AGND, separatefrom the logic ground. All other analog groundsand DGND should be connected to this ground. Noother digital system ground should be connected tothis single-point analog ground. The ground return tothe power supply for this ground should be low impedanceand as short as possible for noise-free operation.High-frequency noise in the V DD power supply mayaffect the high-speed comparator in the ADC. Bypassthese supplies to the single-point analog ground with0.1µF and 4.7µF bypass capacitors close to theMAX192. Minimize capacitor lead lengths for best supply-noiserejection. If the +5V power supply is verynoisy, a 10Ω resistor can be connected as a lowpassfilter, as shown in Figure 18.High-Speed Digital InterfacingThe MAX192 can interface with QSPI at high throughputrates using the circuit in Figure 19. This QSPI circuitcan be programmed to do a conversion on each ofthe eight channels. The result is stored in memory withouttaxing the CPU since QSPI incorporates its ownmicro-sequencer.Figure 20 details the code that sets up QSPI forautonomous operation. In external clock mode, theMAX192 performs a single-ended, unipolar conversionon each of the eight analog input channels. Figure 21shows the timing associated with the assembly code ofFigure 20. The first byte clocked into the MAX192 is thecontrol byte, which triggers the first conversion on CH0.The last two bytes clocked into the MAX192 are allzero, and clock out the results of the CH7 conversion.______________________________________________________________________________________ 19
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MAX192 Low-Power, 8-Channel, Serial 10-Bit ADC - Robotics UWA

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