Hi,everybody. The voltage of my signal is of mv level.this signal is the input of ADCs.I want to raise the input impdedance of the signal,then the signal will be isolated from the ADCs.
Simplely,An op-amp(OPA602) is used.
the signal---PIN3(IN+) PIN2(IN-)--PIN5(OUT)
but I found the result is so bad.Such as,when the signal is about
1) You do not want to "raise the impedance of the *signal*. 2) You want the signal to drive a rather high impedance *load*. 3) The output pin of a standard opamp is pin 6; usually pin 5 is reserved for one end of a trimming pot. 4) At least you picked a JFET input opamp, which gives the highest input impedance. BUT the offset voltage can be as much as 0.25mV, and add that to (about) 1pA times the input resistance, and one may get a total offset near that 1mV you see. 1Kmeg times 1pA gives 1mV; warm the opamp up by 10C and that doubles. If the opamp draws a fair amount of power, it can warm itself up by quite a bit. So, a 200Meg input resistance and a fairly warm opamp could give that result. Add a trimpot...
Almost all DVMs have a 10Meg input resistance, so the signal would have to have an equivalent internal resistance of less than 100K for a one percent error, less than 10K for a 10 percent error... Know your equipment...
How do you know that the input voltage did not load down from 3.3 mV to 2.5 mV when you attached your meter? Did you measure the opamp output voltage while the meter was attached to the input?
My point is that if the input is actually at 3.3 mV (but is not being measured) when you are measuring the output at 3.3 mV but is loaded down to 2.5 mV by the meter only when you are measuring the input (and at that moment, the output also follows the loaded input down to 2.5 mV, but is not being measured at that instant), then there may not actually be any difference between input and output at any given instant.
Have you ruled out this possibility? Are you measuring both voltages simultaneously (or applying a dummy load to the input that matches the meter impedance, while you measure the output)?
If you have, then your opamp is not a very good example of the type. If your result is taking place in a simulation, then the simulated opamp is a worst case example, not a typical one.
If you really need a small fraction of a millivolt input accuracy under wide temperature range, you may have to change the opamp to a self zeroing (chopper) type. These can have DC errors down to less than .1 mV.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.