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reading negative voltages with an adc

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rudie

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hello guys,

I have a problem with my adc. I have a ADS7870 as converter. I want to read 6 positive signals and one negative signal. The positive ones are no problem but how can i read a negative signal of approximately -2V.

i hope that someone can help me

greets rudie
 

Use an inverting buffer on negative channel, e.g. just use an OP-AMP in inverting mode and set its gain to 1. This will convert -2v to +2v.
 

thank you for you replay.

If i use a inverter it must be a very accurate inverter because i don't want more inaccuracy than 1mV. I looked for a device that could do that. At analog devices i found one the ADM8660. I the data sheet is stated that the voltage conversion has an efficiency of 99.96%. So does this mean that the total voltage error in the output is 0.04%. Or are there more parameters that i should take into account.

Greets Rudie
 

I am not aware about the ADM8660 so can't comment on that. I feel an inverter should be accurate (what's the use of -ve feedback otherwise, however it will depend on the accuracy of the resistors used to achieve gain of 1. If you using precision resistors it should not be a problem.
 

well, if i use 0.1% resistors i have an inaccuracy of max. 4mV:

Acl=-Rf/Ri -> (10K*1.001)/(10K*0.999)=1.002
input voltage is 2V so the output voltage is 2*1.002=2.004 that is an inaccuracy of 4mV. The ADM8660 doesn't need the gain setting resistors but i am wondering what the "voltage conversion efficiency" means, as i said is was 99.96%. so that would mean that if the input voltage is 2V the output voltage is 2*0.9996=1.9992. The inaccuracy of this is 0.8mV that would be enough. But i dont know if there are more factors that have an impact on the inaccuracy

greets rudie
 

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