Problem with AD8421 with noise spikes

For our products, we use AD8429ARZ to amply small signals at different gain, 1 , 5, or 25 using analog MUX ADG1404YRUZ. The noise density is around 5 nV/sqrt(Hz) at gain of 25 with shorting input. I tried to use AD8421 and AD8221 to reduce power consumption and leakage current, but both of them show spikes (about 20~30nV/sqrt(Hz), around 13Hz and its harmonics) in spectrum which was caused by some periodic spikes in time series. I did many tests (including removing MUX) and finally figured out the reason is that external noise is coupled into AD8421 and AD84221's Rg loop area. But AD8429 does not have this issue. If I directly solder the Rg on pin 2, 3, there are no such spikes. If I move the board to less noise environment instead of the test bench with PC, there are no such spikes. During layout, I already paid attention to reduce the loop area of Rg, the MUX is as close as possible to AD8421 footprint. I read all the support cases related to AD8421 at analog.com, it suggested not to use analog Mux to switch Rg, instead, adding some R on both ends of Mux to reduce capacitance effect on pin 2,3, I will do that for next revision. My question is as following: 1. Each board has identical 6~8 channels with identical layout and parts, all the channels with AD8429 do not show such spikes while channels with AD8421, 8221 have problem. why there is such difference? Is there any way to improve my design/layout so that AD8421 can be used?

  • 0
    •  Analog Employees 
    on Jul 17, 2017 12:33 AM

    Hi John,

    I have few questions and request for me to understand and isolate the problem.

    Can you share or elaborate your setup on testing the noise of your board. Do you have a preamplifier, dc blocking, spectrum analyzer, etc.? Can you also show us the comparison noise plots of the AD8429, AD8221 and AD8421? It would also be helpful if you could share your layout so that we could see and check if there is something that we could adjust or optimize.

    Thanks and best regards,

    Emman

  • Hi, Emman:

        Today I repeated the test in two situation as bellow, Ch1, Ch2 are installed with AD8421, Ch2, Ch4 are installed with AD8429. In Ch1, the gain mux ADG1404 is removed to elminate any possible reason from Mux, and pin 5, 6 are shorted to have R 249 Ohm as gain resistor. For Ch2, Ch3, Ch4, gain mux are used.  The signal then pass differential driver and sampled by 24-bit ADC AD7768 (it is a matured industrial data logger, used over 2 years). The spectrum and time series are displayed using Matlab. 

     

    1. Internal Shorting noise test over the bench: in this mode, the input of AD8421/8429 are shorting by internal input MUX through Pin 6, 7 of ADG5409. From the time series and spectrum, for Ch1 there are noise spikes around 13~14Hz and its harmonics in spectrum caused by spikes in time series. This time Ch2 does not have spikes, but frequently, it has the same spikes as Ch1 in both time series and spectrum. Ch3, 4 do not have spikes, as always.

     

    1. External Shorting noise test over the bench: in this mode, the input of AD8421/8429 are shorting by shorting input using wire (Ch+, -). From the time series and spectrum, for Ch1, Ch2 there are noise spikes around 13~14Hz and its harmonics in spectrum caused by spikes in time series. Ch3, 4 do not have spikes, as always.

     

    From above tests, we can see, with identical parts and layout, AD8429 have not spikes, while AD8421 have spikes when external noise are presented (eg. PC). I previously tested AD8221 with same set up, it also has noise spikes.

     

    Please see attached schematic, layout, spectrum, and time series.

  • for the external shorting, the spikes on all channels are 60Hz and its harmonics which is normal.