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Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Hi guys
I have to amplify a small differential voltage near the top rail.
This voltage has some indeterminate impedance (like between 100k and 1 Meg),
represented by R1 and R2 on the diagram.
I plan to use the dual jfet op amp TL082:
__________
| + Vx - |
|__________|
| |
R1 R2
| |
__ | |
/+ |--' |
,--< | |
| \_-|--, |
| | |
+---------' |
| |
R3 |
| |
+-------------, |
| | |
| __ | |
p- '-| / -|--' |
channel ||---< | |
mosfet ,-| \_+|-----'
|
+--v out
|
R4
|
gnd
V out goes to a voltage-controlled oscillator, and I
want Vout to get within 200 mV of the rails. In the application Vx is
going to be very small (millivolts) and I'll use a lot of gain, so
offset has to be eliminated.
Now the op amp doesn't have any nulling pins. I wouldn't want to
install two pots for two amps anyway. So I was thinking about
nulling it like this:
R5=R3
__________
| + Vx - |
|__________|
| |
R1 R2
| |
__ | |
/+ |--' |
,--< | |
| \_-|------, |
| | |
+----R5-------+ |
| | |
| \ |
| / |
| ,---->\ |
R3 | / |
| | \ |
| gnd / |
| | |
+-------------+ |
| | |
| __ | |
p- '-| / -|--' |
channel ||---< | |
mosfet ,-| \_+|-----'
|
+--v out
|
R4
|
gnd
Is this correct?
I'm also contemplating adding a couple of fixed resistors in series
with the legs of the potentiometer, to improve the resolution of the
nulling adjustment.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Clever circuit! But nulling pins went out with flares and Knight Rider. If the worst-case untrimmed offset of your op-amp isn't acceptable, buy a more accurate op-amp. The LMC6482A might be suitable.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Thanks for the suggestion. I also realized that the op amp needs an output that goes to the rail, because of the voltage follower configuration. The amp you suggested gets to 20 mV of the rail, according to the datasheet. If I want it in DIP I'll have to get the quad, and it won't be the A version. But the datasheet has some instrumentation amp circuits with correction.
Or I could use a charge pump. Then any old op amp would do. Later kell
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diff amp
