Varying positive and negative voltage into positive only

Thread Starter

jacky1997

Joined Oct 30, 2015
37
My input is an analog voltage varying between 0.5uV and -0.5uV, how can I convert this into a positive voltage so that I can measure it with a microcontroller? The voltage will have to be amplified by an opamp for sure, but I don't know how to work with the negative voltage. A negative power supply isn't an option either.
 

ericgibbs

Joined Jan 29, 2010
15,526
Hi jacky,
Do you already any OPA's you could use.?
+/-500nVolt signal would require a very low noise OPA.
E

BTW. what is the signal source.?
 

BobTPH

Joined Jun 5, 2013
4,907
I think you will need an instrumentation amplifier for a signal at that level. After amplifying, you could use a precision rectifier.

Bob
 

ronsimpson

Joined Oct 7, 2019
2,067
What frequency (or time) for the signal? How long from -.5 to + and back to -5.?

Is this signal "DC" as in -0.5uV=cold and +0.5uV=hot or is it AC, where 0V=no signal and +/- 0.5uV=signal?

I looked up the specifications on my HAM radio receiver. It probably can not find uV signals in the noise. Certainly it can not measure accurately. So building a low noise amplifier is hard and there will be a problem with input offset. It will help if we know where this signal is coming from.
 
Last edited:

Thread Starter

jacky1997

Joined Oct 30, 2015
37
Hi jacky,
Do you already any OPA's you could use.?
+/-500nVolt signal would require a very low noise OPA.
E

BTW. what is the signal source.?
No, I haven't checked which one I should use yet and the source is a bit hard to give details. The source is a selfmade heatflux sensor and the voltage across the electrodes vary between the two given voltages depending on the amount of heat.
 

Thread Starter

jacky1997

Joined Oct 30, 2015
37
What frequency (or time) for the signal? How long from -.5 to + and back to -5.?

Is this signal "DC" as in -0.5uV=cold and +0.5uV=hot or is it AC, where 0V=no signal and +/- 0.5uV=signal?
The fastest changing frequency is around 10Hz I believe. The signal is 0V at standby.
 

Thread Starter

jacky1997

Joined Oct 30, 2015
37
Apart from the noise problems, is there any way I can convert the negative into a positive voltage without a negative power supply?
 

panic mode

Joined Oct 10, 2011
2,135
Apart from the noise problems, is there any way I can convert the negative into a positive voltage without a negative power supply?
sure, need a positive voltage reference and a pair of resistors...
just bring signal to R1, connect other side of R1 to both R2 and your analog input. connect other side of R2 to reference voltage.

but your signal is very small and needs amplification
 

ronsimpson

Joined Oct 7, 2019
2,067
All op-amps have "input offset error". Amplifiers are not perfect. Looking at the amplifiers at digikey.com I see the input offset error can be anywhere from 0.1uV to 900mV. This error is a function of price and complexity. It is easy to make an amplifier what does not know where ground is by too much.

I know you will not tell me what the signal looks like. The amplifier could be complex and expense or simple depending on what is really happening.
I am going to make up a signal. It spends much time at 0V. It has about the same energy&time at +0.5uF as it does at -0.5uV. What I am describing is a signal where the average is 0V. In this case we can "ac couple" the signal and thus use low cost amplifiers.

By AC coupling and building amplifiers that only look at 1hz to 20hz we can remove much noise and amplifier offset error. The first component is a capacitor and a resistor that moves the "0V" point to exactly 1/2 the supply voltage of the micro. Now almost any amp will work. We will have several stages of amplifications (along with ac coupling). (Assume 5V supply) 0V=2.5V which is the center voltage of the ADC. +0.5uV=4V and -0.5uV=1V.
ronsimpson
 

ronsimpson

Joined Oct 7, 2019
2,067
I don't see the value in adding hardware to rectify the signal or do any "thinking" in opamps and diodes. The signal is so low there is no problem, just reading the voltage and doing simple math. If you read 30 times a second you should get several readings for the "+" and sever for the "-".
 

crutschow

Joined Mar 14, 2008
29,800
To amplify the signal, you could use a low-noise op amp such as an OP-07 or NE5532 in a AC coupled circuit with a low-pass filter to reduce wideband noise.
 
Top