Driving analog data convertor AD7823 with an op-amp

Hi-

I have moved this question to the ADC Drivers community.  Someone here should be able to assist you.

Thanks,

AndyR

EngineerZone Community Manager 

Thank you.

Thank you Dan,

My main concern about using a single ended op-amp is noise or cross talk from other ground loops using the same ground reference point. Are there any differential-output amplifiers you can recommend that will work well with the AD7823?

Eyal

Hello Eyal,

On the question of using a single-ended vs. a differential-output amplifier, it generally depends on the level of precision you are trying to obtain. Since the AD7823 is an 8-bit ADC, it's LSB size with a  1.2V reference, one-half of an LSB is 2.3mV and its SINAD is 48 dB, so you won’t have any issues finding an amp that meets the noise/precision limits of the ADC.

Below is a list of op amps that you may want to consider, depending on your total set of application requirements. Since the AD7823 is a low-power ADC, I focused on quoting some low-power op amps for you.

Please let us know if you have any further questions.

Best regards,

Dan

Eyal, I think we need to know a little more about your application, in particular your input signal source. In particular:

A) What is the signal range (positive and negative) and impedance?

B) What is the signal referenced to i.e. is it floating, is it ground referenced or does it have a large offset.

The large LSB step size of an 8 bit converter should help you, so you need to focus on ensuring that the error sources for the amplifier are managed.

First of all, thanks for all the responses.

I'm interested in amplifying a positive signal (between 0-50mv) from a similar photoreceiver to this one and would like to be able to utilize as much as I can of the AD7823 resolution with only a single stage amplifier.

The signal I am interested to amplify is the photoreceiver's output which I believe is referenced to its internal ground (It is a double ended output). Unlike the device linked above, my device has a frequency bandwidth of 100KHz, but a similar output impedance of 16ohm.

The AD8601 seems very appropriate from the frequency point view, GBWP of 8MHz. A simulation also shows that a non inverting single supply configuration with a gain of 10 (500mv = utilizing ~45% of the AD7823 resolution) works reasonably well.

I originally had 2 concerns:

1. At a simple non inverting gain configuration of 20 (1v = utilizing 83% of the AD7823 resolution), the  
    amplifier was further away from the ground rail, 9mv @input of 0mv. I know that this is the downside of
    single supply and will apply a positive offset voltage to the input of the op-amp together with a digital offset at the digital receiver which
    interfaces with the ADC.

2. I am concerned that noise or cross talk from the controller of the ADC on the ground plane will interfere
   with the analog input to the op-amp if I will connect the reference of the photoreceiver to the common
   ground. A possible solution maybe using a digital and an analog ground planes connected by a trace on
   the PCB. Or, as I mentioned before, using a differential output op-amp, separating the analog signal from
   the common ground.

Thanks again for trying to help,

I would appreciate any technical advice and will be glad to provide more information if required.

Eyal

Message was edited by: Eyal Niv

Eyal,

I agree with Oliver. Can you tell us more about the input? 

We can certainly recommend a differential receiver amp like the AD8129/30 but you may not need that.

(Oliver: Thanks for your advice)

Dan

I would look at the AD8031, MAX4124 or MAX4125, LT6233, MAX4488, ADA4851 or the LT6202. They will all comfortable provide x20 amplification at 100kHz with at least a factor of 10 excess loop gain.

Instead of making all your signals 0V referenced you could generate a virtual ground reference somewhere between 0.1V and 0.5V above 0V and then configure the front end amp as a differential amplifer rather than a simple non-inverting amplifier. This would raise the opamp output up but a small amount, bypassing the problem of not swinging right down to ground. The ADC negative input would obviously connect to the virtual ground.

Using a differential amplifier arrangement also provides an easy way of removing ground noise between different parts of your system.

Thank you Oliver!

Right now I have this setup, where I use an offset to solve the voltage swing problem:

This works well in simulation. However, if understand correctly, in this setup I am not generating a virtual ground/rail splitter and as such do not have a ground noise clean reference point for the AD7823.

Thanks a lot for mentioning this. I'll do some reading on the differential op-amp configuration, which I am less familiar with, and would really appreciate if you can elaborate little bit more.

Regards,

Eyal