On Jul 29, 4:10*am, Franc Zabkar wrote:
On Mon, 28 Jul 2008 20:52:07 -0400, Dave M
put finger to keyboard and composed:

A transimpedance amplifier is what you need, as Sam suggested. *Here's a circuit
to start with. * You might have to fiddle with exact resistor values, depending
on the Op-amp you use, which isn't critical, except for input offset voltage.
Select a dual Opamp with low offset voltage, such as the Texas Instruments
OPA2227.
Since it only sees DC currents and voltages, there are no issues with frequency
response. *The circuit runs off a 9V battery ...

I don't mean to nitpick your clever circuit, but you've drawn it as
running off *two* 9V batteries. Maybe you should have designated the
battery terminals at V+ and V- rather than +/-9V.





* * * * * +9V
* * * * * *|
* * * * *+-+-+
* * * * *| * |
* * * * *| * | * * *+-------------+
* * *10K | * | * * *| * * * * * * |
* * *1% *| * | * * *| * *+9V * * *|
* * * * *+-+-+ * * *| * * | * * * |
* * * * * *| * * * *| *|\ | * * * |
* * * * * *| * * * *| *| \| * * * |
* * * * * *| * * * *+--+- \ * * * |
* * * * * *| * * * * * | * \U1B * |
* * * * * *| * * * * * | * *------+
* * * * * *| * * * * * | * / * * *|
* * * * * *+-----------++ / * * * |
* * * * * *| * * * * * | /| * * * |
* * * * * *| * * * * * |/ | * * * |
* * * * *+-+-+ * * * * * *| * * *+++
* * * * *| * | * * * * * -9V * * *+
* * * * *| * |
* * *10K | * |
* * *1% *| * |
* * * * *+-+-+
* * * * * *|
* * * * * -9V

Cheers!!!
Dave M

Never take a laxative and a sleeping pill at the same time!!

Wise words.

- Franc Zabkar
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Great replies: thank for the thoughts and information.
Also for the unit specification; had no indication is was 25 mhz. up.
But the small size/length of the transmission line unit inside it
might have been some indication.
Seems to boild down to a) Look at and modify/jigger the values of the
resistors in the DC part of the switching circuit to increase meter
deflection 'as is'.