How to measure a single-ended +/-10V signal with the AD7768-1?


I have a DC-coupled +/-10V single-ended signal (max. 200kHz) from a high impedance source that I want to measure with the AD7768-1.

Now I wonder about two things:

(1) which will be the best OP-Amp solution to down-scale the single-ended +/-10V input signal to the ADCs (differential) input voltage range and maintain high input impedance (>1M Ohm)?

(2) Should I power the ADC's AVDD1/AVSS pins from a +5V power supply or do I achieve better performance when powering it from a split +/-2.5V power supply, as described in the AD7768-1 data sheet, page 41.

I am looking forward to your answers and suggestions.

Regards, Niels

added "" for input impedance
[edited by: Niels at 1:04 PM (GMT 0) on 22 May 2019]
  • Hi Niels,

    Please see this post which has some relevant information. 


    I would suggest looking at the LTC6363 to drive the ADC.


    The option for a single 5V supply vs +/-2.5V supply should not affect the noise provided the supplies are clean in both cases. It comes down to which option is most convenient, what supplies are available, what does the input signal look like etc. Fully differential amps like LTC6363 have a VOCM pin which can be used with the VCM from AD7768-1 to make this easier.



  • Hello Niall,

    I worked through the post you suggested.

    The LTC6363 (a quite impressive part) with gain of 0.5 converts my single ended +/-10V input signal int o a +/-5V (differential) signal. The AD7768-1 is principally capable to convert this signal range but it's "on  the edge", and I need +6V and -1V power supplies for the LTC6363.

    What about the AD8475? It has a gain of 0.4 and would convert my +/-10V input signal into a +/-4V (differential) signal for the AD7768-1. This can easily converted using a 4.096V reference and a single +5V power supply for OP-Amp and ADC.
    Especially the single +5V supply eases my PCB layer stack and allows me to use more ground-layers for signal separation.

    Will the AD8475 limit the AD7768-1 performance?

    What do you suggest as high impedance input buffer,
    a PGA like e.g. the AD8250 or a low noise OP-Amp?

    Unfortunately I will not have enough PCB area to place a 4 additional OP-Amps, implementing a 8th order Low-Pass-Filter.
    What is your LPF suggestion that requies minimum PCB area and does not degrade AD7768-1 performance?
    (AD7768-1 will alwys run at full speed modulator frequency of 8.192MHz)

    Regards, Niels

  • Hi Niels,

    If +6V and -1V supplies are a concern could you use a gain of 0.4 using discrete resistors and the LTC6363? The bandwidth of the AD8475 will likely be too low to settle the effects of the switching from the modulator of the AD7768-1. 

    I would also look at the harmonic distortion performance of the AD8475 vs the LTC6363. It might be possible to use an amplifer like the ADA4610 followed by the LTC6363/ADA4945 to drive the AD7768-1. 

    One thing to point out which may not have come across clearly on the suggested post, an 8th order antialiasing filter in not required. Take a look at the AD7768-1 datasheet, figure 93, for details of the low pass filter required. The digital filter rejects alias tones and the user only needs to take care of images relating to the modulator frequency. A 3rd/4th order filter is sufficient, provided the selected amplifier has enough bandwidth to provide the required rejection. 2* Fmod is the frequency at which the digital filter provides no protection.



  • Hi Niall,

    sorry for my late response, but I did not get an e-mail alert for your post.

    When using LTC6363 with discrete resistors, what about the (unwanted) effects that acome accross with resistor mismatch?

    What resistor tolerance and temperature drift performance would be necessary? Are 0.1% resistors with TK50 sufficient?

    Regarding the AD8475 bandwidth: did you consider that the AD7768-1 has input precharge buffers that ease the driving requirements of the external OP-Amp?

    Regards, Niels

  • Hi Niels,

    Certainly you will need to consider resistor mismatch and temperature drift. I would recommend resistor tolerance of 0.1% as being the worst case, you may find that your application requires greater precision. For the temperature drift, some factors to consider would be the temperature range of the end application, and whether you can calibrate or temperature compensate in some way?

    Even with the precharge buffers enabled, the AD8475 may struggle to settle the AD7768-1 input when the AD7768-1 is running at the fastest Fmod rates. At lower Fmod rates you will find that the AD8475 can drive the AD7768-1 input with no adverse settling effects.