AD624 In many applications complex software algorithms for autozero 50 ⍀ +INPUT(–INPUT) applications are not available. For these applications Figure 42 provides a hardware solution. G = 100AD624225.3 ⍀ +VSG = 2004445.7 ⍀ VB124 ⍀ 10k ⍀ 1516RG1G = 50080.2 ⍀ 20k ⍀ 10k ⍀ 14RGAD624V1OUTVOUT9 10130.1 F LOWRGCH220k ⍀ LEAKAGE10k ⍀ 10k ⍀ RG21k ⍀ 50 ⍀ –INPUT–V12 11(+INPUT)SAD542+VS1/2AD712DAC AVDDDATADB0VAD7510DIKDSSINPUTS256:1DB7GNDCSAD7528A1A2A3A4WR200 sDAC A/DAC B1/2ZERO PULSEDAC BAD712 Figure 42. Autozero Circuit The microprocessor controlled data acquisition system shown in Figure 43 includes includes both autozero and autogain capabil- Figure 40. Programmable Output Gain Using a DAC ity. By dedicating two of the differential inputs, one to ground and one to the A/D reference, the proper program calibration AUTOZERO CIRCUITS cycles can eliminate both initial accuracy errors and accuracy In many applications it is necessary to provide very accurate errors over temperature. The autozero cycle, in this application, data in high gain configurations. At room temperature the offset converts a number that appears to be ground and then writes effects can be nulled by the use of offset trimpots. Over the that same number (8 bit) to the AD624 which eliminates the operating temperature range, however, offset nulling becomes a zero error since its output has an inverted scale. The autogain problem. The circuit of Figure 41 shows a CMOS DAC operat- cycle converts the A/D reference and compares it with full scale. ing in the bipolar mode and connected to the reference terminal A multiplicative correction factor is then computed and applied to provide software controllable offset adjustments. to subsequent readings. +VS–INPUTRG2RGV1REFG = 100AD583AD624VVG = 200OUTAD7507AD624IN AD574AG = 500AGNDRG2RG+INPUT1A0 A2–VS39k ⍀ VEN A1–VREFREF–VS20k ⍀ 20k ⍀ +VAD589SR320k ⍀ R5R20k ⍀ MSBFBAD7524LATCHDATAOUT1C1+VSR41/210k ⍀ INPUTSLSB10k ⍀ AD7121/21/2AD75245kAD712 ⍀ AD712CSDECODE1/2WROUT2AD712R65k ⍀ CONTROL–VSGNDMICRO-ADDRESS BUSPROCESSOR Figure 41. Software Controllable Offset Figure 43. Microprocessor Controlled Data Acquisition System –12– REV. C
