LTC2380-24 Multiplexing

Hi Vitor,

To deal with the bipolar signal, I would suggest using a fully differential amplifier such as the ADA4945-1. The amplifier can be used to translate the bipolar signal from your preamp to the signal level required by the ADC. Additionally the ADA4945-1 should be able to drive the cable you describe. 

The MUX you have chosen has much higher distortion than either of the ADCs. If distortion is not important to your application, this may not matter.

You do not describe your application or what specs are important, so it is impossible to pick between the two ADCs. The LTC2380-24 has lower THD and better SNR but the AD7641 has an internal reference.

As for multiplexing, the principal is the same for both ADCs. You need to allow for the input signal to settle after changing channels before doing a conversion. The AD7641 should settle faster because it is only 18-bits but you can get better performance out of the 24-bit LTC2380-24.

Thanks for answering!

I have an AD8139. Do you think that will do instead of the ADA4945? Essentially the signal conditioning step should add a common model voltage to both inputs and scale them down, right? AD8139 datasheet doesn't sate if it's possible to achieve less than unity gain though.

The second suggestion is to use the differential amp to drive the cable, instead of the regular op amp, is that so?

Could you provide a quick schematic for both cases?

The application consists in reading AFM probe signal, it's a sinusoid, and both amplitude and phase are important. If I use LTC2380 I'm going to buy the LTC6655-5 external reference. I currently have a REF5040 around but it's too noisy for 24bit. The internal reference would be handy if I can scale down the signal, otherwise AD9641 would only accept 2Vpp.

Which IC do you recommend for multiplexing? I've thought about providing the option of bypassing the mux for comparison (MAX4618 distortion is about -75dB, that's worse than both ADCs indeed).

AD8139 has worse distortion and noise specs and much higher power dissipation. It will do the attenuation function. You just need to pick the proper Rg and Rf values. I'm not an amplifier or mux expert. I would suggest going to the proper EZ forum to ask those questions.

Thanks, you have already helped a lot, anyway!

If possible, please move this thread to the right forum.