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AD8429 voltage noise

Question asked by bnevins on Aug 21, 2015
Latest reply on Nov 9, 2016 by meehanchr

Is the maximum input voltage noise 1nV/rthz?

 

In the data sheet, the 1nV looks like it is in the Max column (page 3), but I want to be sure

that somebody didn't press the tab key accidentally.

 

What are other people getting?

 

I am measuring about 1.4nV /rthz, and it could be several things:

 

0)  Measurement system has a problem.

 

1) PCB inadequate--two layer board, no ground plane

(though at 30khz, should it matter that much?)

 

2) parts damaged by Digikey or me. 

( I use an esd strap while soldering, but lots can go wrong)

 

3) I misunderstand the data sheet, the input noise can be greater than 1nV/rthz

 

4)  Digikey bought parts from a 3rd party wholesaler, and said parts are ADI rejects.

(This was said to be a big problem at one company I worked at).

 

 

Measurement Setup:

 

The AD8429 has gain of 1000, and I connect the output to another x100 amplifier, then into DSO/FFT.

 

 

To verify my system I do these tests:

 

0) measure supply voltage noise:  it seems low enough not to matter. (PSRR is high on AD8429)

 

1) ground input to 2nd amplifier.  Output signal is much smaller than when AD8429 is connected.

 

2) place Resistors from 0 ohms to 250 ohms in series with ad8429 inputs, measure the resulting power density.

From this, the system gain can be calculated and the amplifier noise compared to that of a resistor.

The amp noise is showing the power of a 100 ohm resistor.  It should show the power of a 60 ohm resistor.

 

To Resolve this,  I plan these steps:

 

1) order an eval board and 5 pieces directly from ADI.

 

2) build a 2nd amp right on the board so it can be connected directly to DS0/FFT

 

3) power the board from 9V batteries to minimize supply noise.

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