Tough TIA

Family. Hiking. Cooking. Eating. I need an electronics break.

I'm thinking about a product that measures large (as in 60 feet tall)
capacitors, and it needs a tough, hard to damage TIA. AC coupling is
fine, and the range is basically audio.

Something like this:

https://www.dropbox.com/s/mrf7kbvtboktstz/Tough_TIA.jpg?raw=1

One could close a fancy bias servo loop on Q1, but it's hardly worth
the trouble. R1 could be a proper current sink too, I guess. 

If I run Q1 at, say, 5 mA, the diode impedance looking into the
emitter of Q1 is about 5 ohms, so I want a transistor that doesn't add
a lot of emitter resistance. 

After the food comas resolve, I guess I could Spice it.

On Thursday, November 24, 2022 at 1:49:09 PM UTC-8, John Larkin wrote:
> Family. Hiking. Cooking. Eating. I need an electronics break. 
> 
> I'm thinking about a product that measures large (as in 60 feet tall) 
> capacitors, and it needs a tough, hard to damage TIA. AC coupling is 
> fine, and the range is basically audio. 
> 
> Something like this: 
> 
> https://www.dropbox.com/s/mrf7kbvtboktstz/Tough_TIA.jpg?raw=1 

Looks good, but... your input clamps can go through an inductor to ground  instead of
straight to ground; the input capacitor is assurance that the inductor
doesn't saturate.   That''ll  help stretch out the protection diode currents when lightning strikes.
Neon light would be best clamp for that kind of zap... put it
outboard of C1.

And, the C2 is redundant if  you drop the Q1/Q2
level translation (why use capacitors AND level translation?).

Also, if it matters, a duplicate resistor to R3 can go from (+)  input to
ground, just to keep input impedances DC-balanced.

On Thu, 24 Nov 2022 20:14:31 -0800 (PST), whit3rd <whit3rd@gmail.com>
wrote:

>On Thursday, November 24, 2022 at 1:49:09 PM UTC-8, John Larkin wrote:
>> Family. Hiking. Cooking. Eating. I need an electronics break. 
>> 
>> I'm thinking about a product that measures large (as in 60 feet tall) 
>> capacitors, and it needs a tough, hard to damage TIA. AC coupling is 
>> fine, and the range is basically audio. 
>> 
>> Something like this: 
>> 
>> https://www.dropbox.com/s/mrf7kbvtboktstz/Tough_TIA.jpg?raw=1 
>
>Looks good, but... your input clamps can go through an inductor to ground  instead of
>straight to ground; the input capacitor is assurance that the inductor
>doesn't saturate.   That''ll  help stretch out the protection diode currents when lightning strikes.
>Neon light would be best clamp for that kind of zap... put it
>outboard of C1.

I want this to be intrinsically safe in the outgoing direction too;
the capacitance I'm measuring might be gasoline or something. Might
need four diodes for super redundant safety.

>
>And, the C2 is redundant if  you drop the Q1/Q2
>level translation (why use capacitors AND level translation?).

C2 keeps DC out of the opamp, the actual TIA. I like the transistor
because it should be very stable and isolates the opamp loop from the
impedance variations of the incoming current.

>
>Also, if it matters, a duplicate resistor to R3 can go from (+)  input to
>ground, just to keep input impedances DC-balanced.

Modern opamps have pA bias currents, or fA. There's seldom need to
balance input resistances. Haven't done that in ages.

The opamp output will get digitized and go into an equivalent phase
detector/lockin amp, so a little DC offset actually doesn't matter.

I'm still playing with the idea... nothing's final. Basically I want
to do a 3-terminal measurement of a concentric-tube capacitive liquid
level sensor. Apply a sine wave to the outer tube and measure the
current induced into the inner with this sort of TIA. Long cables
might be involved.

Thanks for the comments. It's good to think things over different
ways.