differential amplifier

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.

.

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