Re: TM500 'specials' & OEMs


On Fri, 30 Sep 2016 12:43:54 +0100, you wrote:

from Tim P (UK)

I saw the two 'specials' on Christian's website - the ZIG-T is interesting
in that ultra-high impedance amplifiers have application in Cell Physiology
for use with micro-electrodes. I'd love to see the schematic.


The input amplifier will be about what one would expect; The ZIG-T was
recent enough to use JFET or MOSFET inputs without a shunt resistor.
See the 7A13, 7A22, and DM502 schematics for examples where the input
shunt resistor can be disconnect.

It is easy to get into the 10s of gigohms range and much higher today
using a JFET or MOSFET input operational amplifier. Check out the
common mode input resistance specification on an operational amplifier
datasheet which applies to the non-inverting input; for an LMC6081, it
is greater than 10 teraohms (10 mega megohms) typical. DIP parts have
an advantage here over surface mount because the non-inverting input
pin can be bent up away from the printed circuit board and wired in
the air which is an excellent insulator.

Close attention needs to be paid to input leakage from other sources
including input protection circuits.

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