[time-nuts] Fury Interface Board: How about TI OPA277?

xaos at darksmile.net xaos at darksmile.net
Thu Nov 1 23:09:29 EDT 2007

It is funny how things you forgot a long time ago have a way of coming back.

Case in point. I was just running some simulations on the EFC  
interface and then I had this idea.

Wouldn't an instrumentation amplifier be the right way the isolate the  
Fury EFC    voltage?

For instance, the Burr Brown INA 103. It would provide for isolation  
as well as gain. Not sure of all the details yet.

I remember using this device for a thermocouple a few years back. And  
I even managed to find a few of them in a box somewhere.

What do you guys think?

> LT1085 regulators are probably too noisy even when configured correctly.
> The output voltage tempco is probably a little high also.
> OCXO requires a supply voltage with better than 0.01% stability for time
> intervals less than the discipling loop response time.
>> - Wouldn't it make more sense to configure U1A and U1B as simple
>>   non-inverting amplifiers, that will present a high-impedance load to
>>   EFC_input, thereby avoiding the need for all the J8-J10 and J21-J23
>>   jumpers?  You need a 3.3V reference for the U1A circuit rather than
>>   1.5V, but that's easy enough to make out of a TL431.

This was my mistake. The device should've  been the LT1084-12. The  
specs are not better though.

> Not really practical if one needs to compensate for small voltage
> differences between the Fury EFC gnd and the OCXO EFC gnd,
> However with a suitable star ground and independent supplies for the
> Fury and the interface board this should not be an issue.
> There is little point in using a low drift opamp with a TL431 reference.

There is no question that he TL431 is not right here. I am already  
running the simulation with an LM128 or such. PSPICE only has the  
LM128 so I am just testing this out.


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