Amplifier Noise

Oct 06, 2005 3 Replies

Hello ,



Please go to the following link



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and then go page# 13, figure#16. I tied pin#2 of INA133 to +15volts through a 2MegaOhm variable resistor. pin#3 is connected to a voltage ouput DAC ( AD5532HS ) producing a sine wave of 0 to +5volts unipolar with a offset of zero volts. I adjusted +2.5 volts at the pin#2 inorder to make the sine wave bipolar ( +2.5 volts to -2.5 volts peak to peak). There is a capacitor ( 0.1 uF) between the load and OPA131's non-inverting input inorder to stop DC going to the load. I added the variable resistor so that I can adjust the voltage at the input of the capacitor. Because the capacitor was getting biased with the -15volts supply( When there was no output from the DAC). But now when DAC is not giving any signal to the INA133 ( ZERO) the INA133 is producing a noise or sine wave kind of signal and its also passing through the capacitor to the load. The pin#5 of the INA133 is connected to pin #6 of the OPA131. The Reference pin ( pin#1 ) of the INA133 is connected to ground. The output pin#6 of INA133 is connected to the resistor "R". My load is resistive. My supply is plusminus 15 volts. My variable resistor is surface mount one turn variale resistor.



Regards John



I read in sci.electronics.design that john wrote (in ) about 'Amplifier Noise', on Thu, 6 Oct 2005:

Don't. Put a 10 kohm pot between +15 and ground and connect the slider to pin 2. Even better, put a 22 kohm resistor between the +15 V and the hot end of the pot, so that 2.5 V happens with the slider near the middle of its range.

If you do that, a 0 V signal from you DAC will give you maximum negative output current. Is that what you really want?

Have you got any DC path from the + input to anywhere? If not, the op-amp will not work.

If you really do want a bipolar current, I suggest you don't bias up the in-amp to get it, but accept a unipolar output from your current

-converter and then convert it to bipolar in a further stage. That eliminates the capacitor you put on the + input of the op-amp.

So you've grounded the feedback signal from the op-amp!

How do you come to have this task? It is clearly taxing your understanding a great deal.

Regards, John Woodgate, OOO - Own Opinions Only. If everything has been designed, a god designed evolution by natural selection. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

selection.

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