Differential ADC Biasing Techniques, Tips and Tricks

1 2002 Microchip Technology 1 MAN842 INTRODUCTIONTrue Differential converters can offer many advantagesover single-ended input A/D Converters (ADC). In addi-tion to their common mode rejection ability, these con-verters can also be used to overcome many DC biasinglimitations of common signal conditioning below are some typical application issues thatcan be solved with proper Biasing of a Differential con-verter: Limited output swing of amplifiers Unwanted DC-bias point Low level noise riding on ground Unwanted or changing common mode level of input signalThis application note discusses Differential input config-urations and their operation, circuits to implementthese input modes and techniques in choosing the cor-rect voltage levels to overcome the previouslymentioned AND SINGLE-ENDED INPUT CONFIGURATIONSB efore discussing Biasing solutions, it is important tounderstand the functionality of Differential A/D convert-ers.

2 The true Differential A/D converter outputs a digitalrepresentation of a Differential input signal, typically atwo s complement binary formatted output. The con-verter output can be either signed positive or negative,depending on the voltage level of the Differential following equation expresses this relationship forthe MCP330X devices:EQUATION:The binary output for the MCP330X is a 13-bit output(12-bit plus sign output). It is important to note that the converter output is zerowhen the inputs are equal. As the voltage differencebetween IN+ and IN- increases, the output code alsoincreases. The maximum voltage at which digital codesaturation will occur is VREF. The Differential conver-sion of the MCP330X converters will reject any DCcommon mode signal at the inputs.

3 For the MCP330 Xconverters, the common mode input range is rail-to-rail, to VDD+ circuit in Figure 1 shows a Differential signal beingapplied to the IN+ and IN- pins of the converter. Thismethod is referred to as full Differential operation of theconverter. The graph below the circuit shows possiblevoltage levels for a Differential application. The inputsare centered around a common mode voltage, is equal to the maximum input swing, shown hereas VDD. By setting VREF equal to the maximum inputswing of the signal, the full range of the A/D converteris being used. FIGURE 1:Driving a true differentialconverter with a true Differential :Craig L. KingMicrochip Technology Code2n()IN+IN- ()2 VREF------------------------------------ --=VDD1 FInput SignalDifferentialVREFp-pVREFp-pVCM-4096 +4095 Output CodeIN+IN-VREFVDD1/2 VDDGNDV oltage Levels (V)VCMIN+IN-VREFVDDVSSD ifferential ADC Biasing Techniques, Tips and TricksAN842DS00842A-page 2 2002 Microchip Technology SIGNALSSome signals are single-ended, and a true differentialconverter can be used in this situation as well.

4 Figure 2shows a single-ended signal being applied to the IN+terminal. The common mode voltage is connected tothe negative input of the A/D converter, with the signalconnected to the positive input. This method is referredto as pseudo- Differential operation, with only one of theinputs being used to obtain a bipolar output of graph below the circuit in Figure 2 shows that bysetting VREF and IN- to half of the input swing of the sig-nal, all codes will be present at the output. (Thenumbers shown in this example are for a 13-bitconverter). FIGURE 2:Driving a true differentialconverter with a single-ended input to obtainbipolar output Differential Biasing CIRCUITS FOR SINGLE-ENDED APPLICATIONSIn most applications, the voltage reference of the ADCwill be the most stable voltage source in the accuracy of your data acquisition system is nomore accurate than the voltage reference for the con-verter itself.

5 This same reference should be used asyour DC bias point in pseudo Differential 2 shows that with a single-ended input, the IN-and VREF need to be near the midscale of the signalinput swing. An example circuit using this approach isshown in Figure 3. For a signal with a 5Vp-p swing, IN-and VREF need to be biased at FIGURE 3:Example of pseudodifferential Biasing MCP1525, voltage reference was chosenwhere no greater than 1% initial accuracy or 50 ppmtempco is required. This reference voltage is drivingthree nodes of the circuit: the VREF for the converter,the common mode signal of the signal and the DC biaspoint of the signal input going into the positive channelof the A/D converter. With capacitor C1, AC-couplingVIN, we are effectively blocking any DC component ofthe input signal.

6 This allows us to regulate the DC biaspoint and match this voltage to the common modevoltage and A/D voltage this case, VREF, IN- and VCM have been adjusted toappropriate levels, but still limits the effective inputrange of the converter. This assumes that the outputswing of the amplifier is ideal ( rail-to-rail). In realworld applications, this output swing will be limited bytens or hundreds of millivolts, depending on the outputswing of the Differential Biasing TIPS & TRICKSIn choosing the correct VREF and IN- levels, the outputswing limitations of the amplifier can be overcome. Theobjective is to bring the input range of the ADC awayfrom both supply rails. To move the ADC input rangeaway from the upper supply rail, VREF needs to beslightly less than VDD/2.

7 To move the ADC input rangeaway from the lower supply rail, IN- needs to be slightlygreater than VREF. How far away from the supply railsdepends on the output swing of the amplifier. Figure 4shows this situation +4095 Output CodeIN+IN-VREFVDD1/2 VDDGNDV oltage Levels (V)VDD1 FInput SignalSingle-EndedVREFp-pIN+IN-VREFVDDVS S1/2 VDDVDD1 FMCP60110 FR4R3R1C1 VIN-+MCP1525 VINVOUTIN+IN-VREFMCP330X 2002 Microchip Technology 3AN842 FIGURE 4:Actual input showingamplifier the circuit of Figure 5, a VREF is used to supplythe reference voltage for the converter. The objectivehere is to limit VREF<VDD/2, keeping the required highside output swing of the amplifier less than the upperrail. The IN- is biased at , slightly above VREF. Thiskeeps the required low side swing of the amplifier awayfrom the rail.

8 R3 and R4 are chosen to gain the signal tothese levels, which are now within the output swingcapability of the amplifier. With this configuration, theentire output range of the A/D converter is being applications requiring greater precision, a VREF might be required, instead of the voltagedivider shown. FIGURE 5:Circuit solution to overcomeamplifier output swing MODE VS. VREFFrom the equation on page one, it can be seen that dig-ital saturation occurs when the difference of the inputsis equal to or greater than the voltage reference. Inorder to avoid this and maximize the input range of theADC, care should be taken in setting the commonmode voltage for both pseudo Differential and true dif-ferential input range of the MCP330X devices is slightlywider than the power rails: to VDD+ Therange of the VREF is 400 mV to VDD.

9 These two con-straints, along with the two methods of driving the input,provide specific ranges for the common mode 6 and Figure 7 show the relationship betweenVREF and the common mode voltage. FIGURE 6:Common Mode Rangeversus VREF for True Differential Input mode. FIGURE 7:Common Mode Rangeversus VREF for Pseudo Differential Input CodeIN+IN- > VREFVREF < VDD/2 VDD1/2 VDDGNDV oltage Levels (V)+4095 High side rail limitation of amplifier output swingLow side rail limitation of amplifier output swingVDD = 5V1 FMCP60110 FR4R3R1C1 VIN-+REF191 VINVOUT10 F10 k 10 k IN+IN-VREFMCP330 XVREF (V) = Mode Range (V)VREF (V) = Mode Range (V)3AN842DS00842A-page 4 2002 Microchip Technology smaller VREF allows for wider flexibility in a commonmode voltage.

10 It should be noted however that bydecreasing the VREF, linearity performance is sacri-ficed. Characterization graphs for Microchip s true dif-ferential ADCs show this relationship. These graphscan be found in all MCP330X data sheets. Figure 8shows an example graph, showing slight degradationin INL at lower voltage references. It is specified that novoltage lower than 400 mV should be used as VREF forthe MCP330X 8:Converter linearity is notsacrificed at lower voltage references, down to400 mV. The pseudo Differential method of driving the ADCusing only one input as a signal input limits the VREF range to A reference of larger than wouldrequire that the input swing of 2*VREF be larger thanVDD max of 5V in order to exercise all possible input configurations for truedifferential converters is essential to maximizing theirfunctionality.