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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Nov 1 20:28:04 EDT 2007

kevin-usenet at horizon.com wrote:
> Vos is 1 uV/C guaranteed, 0.15 V/C typical.  (And you can get a
> high-spec part with the latter guaranteed.)  Can take 24 V supply,
> and pull to within 1.5V of the rails over the full temperature range,
> so can produce +/-10V.
> Also pretty cheap.
> Even at 0.1 ppm/V, that would be a worst-case 1e-13/C tempco, which is
> comparable to the best double-oven oscillators, and it'll usually be
> much less than that.
Flicker noise is probably a little high.
> Of course, the tempco of the resistors in the circuit is also an issue.
> I haven't done a full sensitivity analysis, but do know that thin-film
> resistors usually track very well, so as long as you can keep them
> isothermal, it'll work well.
> One trick I use to get better tracking is to build ratios out of multiples
> of a single resistor value.
Can you guarantee this works every time?
> As I said in my other mail:
> - I really fail to see how the LT1085 power supply circuits work at all.
>   An LT1085 is an adjustable three-terminal regulator with a reference
>   voltage (where it tries to keep Vout - Adj) of 1.25V.  U4 will keep
>   the Vcc net 1.25V above Gnd, and U5 will do just plain strange things.
>   Having Vin lower than Vout will turn on internal diodes and short
>   them together.
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.
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.


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