Input signal on ADS7854 - Page 1
Forum section: electronics / projects

author: Hawaka
author_rank: Regular Contributor
timestamp: January 30, 2022, 01:04:32 pm
content: Hello,For a project I'm planning to use an ADS7854 ADC, and I'm not sure how to connect the diff input.For side A, I'm coming straight out of an AMC1350, so I went straight to the input (V+ to AINP_A and V- to AINM_A). I believe this is ok. I also added a antialiasing filter in-between.For side B, the signal is coming out of an AD8750. Can I got into the ADC in pseudo-diff (OUT to AINP_B and REF to AINM_B)? The REF voltage would be 2.5V DC. Also with a antialiasing filter in-between.Everything is in 5V, except the AD8750 in ±5V. I know that the specs of the ADC says full diff only, but in terms of voltage I believe I'm still in specs. Would that works?Thanks in advance for any advise,Hawaka

author: moffy
author_rank: Super Contributor
timestamp: January 30, 2022, 10:50:12 pm
content: The AMC1350 is rated in the datasheet for CLoad(OUTP-OUTN) of 250pF so I don't think it will drive the output filter, it might with the 15R isolation resistors but you'll need to verify that. The VOUTcm voltage is 1.44V so that should work pretty well with the 2.5V reference of the ADC.The AD8250 should work as long as the ADC is in 2xVref mode, whatever that is, but will that affect the AMC 1350 input as well?Looks like the AD8250 needs a minimum of 100R resistor to isolate it from the capacitance of a 1nF load don't know what is needed for a 33nF load.

author: moffy
author_rank: Super Contributor
timestamp: January 30, 2022, 11:13:57 pm
content: Sorry, the AD8250 input is not true differential, not recommended.Why don't you use the ADS7853 which can deal with pseudo differential and single ended inputs instead? Make your life easier.

author: Hawaka
author_rank: Regular Contributor
timestamp: February 01, 2022, 08:38:42 pm
content: Thanks for the info moffy.So if I exchange the ADS7854 for the ADS7853 and remove the filter for the AMC1350 I'm good for that channel. I might put the filter on the input side of the AMC, not sure yet. Anyway I plan to run the ADC at max 100ksps, so I think I'll be fine.For the second channel, I'm gonna leave it as it is. ADS7853 has a max of 22R for the filter resistor. I might bypass it and put the filter before the AD8250 tho.

author: moffy
author_rank: Super Contributor
timestamp: February 02, 2022, 12:01:47 am
content: These ADCs, high speed SigmaDelta, require a low resistance moderate capacitance to achieve their performance. For the ADS7853 it can be 4R/1.8nf, 10R/8.2nf etc. check the data sheet. They use the OPA836 as a buffer because it can drive this impedance. You will probably need to buffer both the AMC1350 and the AD8250. This filter has to be directly before the ADC because its input impedance is very dynamic during the sampling/conversion process. Without a proper input filter the noise performance and linearity of the ADC will suffer significantly.

author: Hawaka
author_rank: Regular Contributor
timestamp: February 02, 2022, 08:02:16 pm
content: You're right I shouldn't cut corner. Then I think I'm gonna put an AD8476 in front off the ADC on both channels. That way I can better tune the filter and also keep the ADS7854.

author: moffy
author_rank: Super Contributor
timestamp: February 02, 2022, 09:22:51 pm
content: You might want to consider the THS4521, shown in the ADS7254 data sheet, instead of the AD8476. The AD8476, when driving capacitance, needs a 100R isolation resistor, which is fine for the SAR ADC shown in the data sheet, but way too large for a SigmaDelta ADC which needs somewhere nearer to 10R. They are quite hard to drive and require low resistance and relatively high capacitance, and there are only a limited number of drivers that can handle that. If you tried to use the AD8476 with 10R/2nF you would at best get lots of ringing and more likely the opamp would oscillate. The AD8476 is only rated to drive 20pF directly.

author: Hawaka
author_rank: Regular Contributor
timestamp: February 03, 2022, 08:12:00 pm
content: Thanks for the info, I'll definitely look more into it. Might even change the ADC.On the datasheet of the AD8476, where they show figure 56 the filter BW is 720kHz and 250kSPS for the ADC. But I thought that the filter BW should be at max half the sampling frequency to avoid folding, so how come that be?

author: moffy
author_rank: Super Contributor
timestamp: February 03, 2022, 11:12:39 pm
content: A quote from the datasheet:"The filter between the AD8476 and the ADC reduces highfrequency noise and reduces switching transients from thesampling of the ADC.Choose the values of this filter with care. Optimal values for thefilter may need to be determined empirically, but the guidelinesdiscussed herein are provided to help the user. For optimumperformance, this filter should be fast enough to settle full-scaleto 0.5 LSB of the ADC within the acquisition time specified inthe ADC data sheet, in this case, the AD7687."So this is not your Nyquist filter, but one purely to improve the performance of the ADC.