[sdiy] Large resistor value (LDR) / DIY ohmmeter question

[aankrom]

I am building an analog ohmmeter from a schematic out of Encyclopedia 
of Electronic Circuits. It uses a reference resistor to compare to the 
resistor under test which is in the feedback loop of an op-amp. The 
design goes up to 10MOhm, but I want to be able to measure resistance up 
to 400MOhm. My question is, is there a point at which a high resistance 
will surpass the open-loop gain of the op-amp, in this case an LM741. 
I'm guessing I should use an op-amp with higher open-loop gain which 
would probably be something like an FET op-amp like a TL071 or maybe an 
LF351. Or am I thinking about it wrong?

My main use for the meter is to watch an LDR return to its dark 
resistance after an LED lowers it. I even have a mod. that I added that 
will put the meter in the 10kOhm range when the LED is on and then 
switch to a higher range (20MOhm-100MOhm) when the LED is off. I'm not 
after precision readings here, just the rate of change. I have a 
cool-looking old 1mA meter. I just hope it has a fast enough action to 
track quick changes.

I make LED/LDR optoisolators for various DIY audio gear and I need to 
separate the fast from the slow because I have plans for each. My DMM's 
won't fit the bill because they don't track changing resistance in the 
high range very well. LDR's dark resistance shouldn't be much more than 
100MOhm, but I know it's higher than 20MOhm (the max. on my meter). I 
suppose I could put a resistor in parallel with the LDR and I could 
still track it's rate of change in a lower Ohm setting (like 10MOhm). 
When the meter stops moving, I can roughly calculate the dark resistance 
based on the resistor in parallel.

I doubt I'll need a 400MOhm range, but I thought I might test some of 
the high-voltage parts in my oscilloscope. I actually have some 100Mohm, 
200MOhm & 400MOhm resistors as well.

So I'm still curious if I should use a different op-amp...

AA