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 Subject :measuring small signals.. 2015-08-27 19:25:28 
jdc
Joined: 2015-08-27 09:44:42
Posts: 5
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From the schematics, it looks like the hardware gain is fixed.  So I'm wondering how small a signal can be measured.  The input range is -14V to 20V and so if 8 bit ADC is used, that would give a resolution of 34V/256 = 0.133V, which isn't that good.  I suspect something clever is done to improve this.  Are additional bits used?  Or something else?  Can low voltage signals be measured?

Thanks!

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 Subject :Re:measuring small signals.. 2015-08-31 18:15:13 
ganzziani
Joined: 2012-04-02 15:42:39
Posts: 463
Location: Sarasota, FL
 
The microcontroller itself has an amplifier on the ADC, with multiple gains up to 64x.
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Gabriel Anzziani
 Subject :Re:measuring small signals.. 2015-09-01 14:31:53 
jdc
Joined: 2015-08-27 09:44:42
Posts: 5
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Thanks, that explains it. I got my xminilab yesterday and I did some tests using a voltage divider driven by the AWG output.
With a signal of about 0.04V peak to peak, the waveform is quite noisy, so that seems to be about the limit of the device.
Are there any tricks for measuring a smaller AC signal?
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 Subject :Re:measuring small signals.. 2015-09-03 14:23:42 
ganzziani
Joined: 2012-04-02 15:42:39
Posts: 463
Location: Sarasota, FL
 

To get a 100x increase in the gain:

1) Swap R2 (820K) and R3 (180K).
2) Replace R7 (20K) with 110K
3) Replace R8 (180K) with 4.7K

To understand how to calculate the gain:
XScopes Analog Front End

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Gabriel Anzziani
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