AD4020 in single-ended operation

Question

Recently got my EVAL-AD4020FMCZ. Want to use AD4020 for single-ended conversions within [0...+5] V, but i'm not sure if i can make it without using ADA4940 or similar. 
 What is the reason analog inputs beeing pulling up to midscale value of reference voltage? 
 Can i make schematics for single-ended conversions like in picture below?

tschmitt · Accepted Answer

Thanks for your query. 
 The AD4020 has an input common-mode voltage requirement which requires that the average voltage of the IN+ and IN- pins be nominally VREF/2 (+/- 100mV). 
 The proposed single-ended circuit would unfortunately violate this requirement, as the common mode would end up spanning from 0 to VREF/2. 
 This is why the EVAL-AD4020FMCZ board is configured by default with VREF/2 driven to the inputs of the ADA4807-1s driving the AD4020 inputs. 
 A single-ended to differential conversion can be implemented either with two op amps or with one fully-differential amplifier (FDA). 
 The op amp option is pictured in Figure 76 of the ADA4807 data sheet. The single-ended input signal is applied to the input of a non-inverting op amp, and the antiphase signal (the other "half" of the differential output signal) is generated by applying the output of the non-inverting amplifier to the input of an inverting amplifier with gain of -1. This configuration is also supported by the EVAL-AD4020FMCZ board by modifying the on-board passive components (most notably, R87 acts to feed the output of U12 to the inverting input of U14). 
 The FDA option is a simpler circuit with fewer components but unfortunately the recommended ADA4940-1 (U2) is not populated on the EVAL-AD4020FMCZ board by default. Personally I would recommend prototyping with the AMC-ADA4945-1EBZ board, which comes populated with the ADA4945-1 with feedback networks and appropriate VOCM voltage already generated by default. This board slots into J8 and J9 headers to bypass the amplifiers populated on the EVAL-AD4020FMCZ board. Refer to Table 4 in the EVAL-AD4020FMCZ User Guide for instructions on configuring the EVAL-AD4020FMCZ board to work with the AMC-ADA4945-1EBZ board. 
 The op amp approach can also be implemented on the op-amp compatible AMCs, which are described in UG-1224 . 
 The main differences between the two architectures are that the FDA option is simpler with fewer active components, but it does not provide a high-impedance input, since the signal source will be driving its feedback network instead of a high-impedance op amp input. 
 Let me know if you have further queries. 
 -Tyler)

Electronic Design Services - PartnerZone

Products Mentioned

AD4020 in single-ended operation