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

Fri Nov 2 19:42:14 EDT 2007

Bruce Griffiths wrote:
> kevin-usenet at horizon.com wrote:
>   
>>>> 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?
>>>     
>>>       
>> Define "guarantee".  For a reasonable risk ratio, yes.  Resistor
>> manufacturers don't go out of their way to package resistors from
>> different batches on the same reel, so you if you use adjacent parts from
>> the reel, the odds are overwhelming that you'll get a uniform material
>> composition, which leads to a consistent tempco.
>>
>> That's certainly good enough for me to build a product out of
>> the parts before doing the temperature testing.  (It's not
>> like parts always meet guaranteed specs, either.  Remember the
>> Signetics 25120 Sale price vs. AQL curve.)
>>
>> If you want it pre-tested, get a resistor network.  Either custom, or
>> again get multiple copies of the same resistance and connect them in
>> the ratios you want.
>>
>> For more on the subjet of resistor tempco tracking, including all
>> of the above suggestions echoed by a moderately experienced
>> engineer, see "Debugging Analog Circuits" by Robert A. Pease.
>> See chapter 2, page 28.
>>
>>   
>>     
> Even better use a device with built in trimmed resistors.
>   
>>> 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.
>>>     
>>>       
>> Huh?  Where do you get that from?  Dont they internally subregulate?
>> The raw power is used only for the oven... oh.  Yes, and power is V^2/R,
>> so epsilon change in voltage causes 2*epsilon change in oven power.
>>
>>   
>>     
> Try actually reading the OCXO datasheet rather than speculating.
> The simple zener regulator built into the OCXO has finite rejection of
> input supply fluctuations.
> The output buffer stage is supplied directly from the voltage at the
> OCXO pins, it has no internal voltage regulator.
>   
>> Which boils down to a statement that the OCXO should internally
>> subregulate.  (With possibly a hack to bypass the regulator during
>> warm-up.)  Does anyone know how this works?
>>
>>   
>>     
> Stop speculating and look at the actual circuit schematics for the
> 10811A and the 10544A.
> neither the oven controller nor the oven heater are powered from an
> internal regulator.
>   
>>>> - 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.
>>>     
>>>       
>> Ah, I didn't realize that was an issue.  That makes life trickier.
>>
>>   
>>     
> Theres also the voltage drop across the OCXO socket contacts with the
> 10811A and the 10544A.
> This will be modulated by the oven current variations.
>   
>>> There is little point in using a low drift opamp with a TL431 reference.
>>>     
>>>       
>> Good point; I just mentioned the TL431 because it was already there.
>> FWIW, the LT1019 is excellent.
>>
>>   
>>     
> If you don't mind its noise.
>   
>> And once you have one precision reference on board, it and a few pass
>> transistors can give you a precision reference for your supplies.
>>
>>
>>   
>>     
>>> Personally, I like the idea of feeding the EFC to a high impedance
>>> amplifier.  However, I am afraid that some would object to having the
>>> EFC go through an op-amp. It will add noise and distortion.
>>>     
>>>       
>> It's going through an op-amp on the Fury board anyway.  If we have any
>> specs on that, we can try to match it rather than overdesigning.
>>
>>   
>>     
> Yes and distortion is virtually irrelevant as the EFC frequency versus
> voltage characteristic is unlikely to be that linear.
>   
>>> I was thinking about a instrumentation amplifier, like the Burr
>>> Brown INA103.  It will provide complete isolation for the EFC signal.
>>>     
>>>       
>> There's no need for a high-impedance input for the reference voltage or
>> input ground, only the input EFC, but it would certainly work.
>>
>> However, the INA103 has, again, a rather high offset tempco
>> (1 + 20/G uV/C) at low gains.  This is because in-amps are generally
>> designed for use at high gains, and so the second stage isn't optimized
>> for low noise.
>>
>> I like the INA337, but it doesn't have the power supply range.
>> The data sheet mentions some other parts... INA114?
>>
>>   
>>     
> The INA114 has much lower bias currents than the INA103 but again has
> high noise and drift at low gains.
>   
>> I'm a little worried about the gain resistor tempco for single-resistor
>> solutions.  (Which is why I'm fond of the INA337.)  But how much of the
>> EFC range is actually used during normal operation?  Does it have to
>> be 1e-13 stable at the extremes?
>>
>>   
>>     
> Typically a good 10811A will achieve an AVAR of a few parts in 1E12 for
> tau> 1sec when being disciplined.
> Its just necessary that frequency fluctuations due to power supply
> fluctuations and other causes should be somewhat less than this for tau
> < disciplining loop response time.
> The disciplining loop cannot correct OCXO frequency fluctuations that
> occur at rates beyond the loop bandwidth.
>   
>>> As far as (J8-J10) only one would be connected at a time. I would not
>>> be feeding all 3 at the same time.
>>>     
>>>       
>> I was just thinking, if they were high impedance, there would be
>> no need to ever disconnect them.
>>
>> (Another question is whether it's reasonable to use the same circuit
>> for the 0-10V and +/-10V options, and just install different components.)
>>
>>   
>>     
>>> It has been made clear to me that the +24V(oven supply), +12V(oven
>>> supply) and +/-12V(EFC interface) supplies must be isolated from each
>>> other.
>>>
>>> The problem is that the oven is very noisy and will corrupt the ground
>>> for everyone else.
>>>
>>> This needs some thinking.
>>>     
>>>       
>> H'm... and if you really want the full +/-10V range, fitting a regulator
>> and op-amp into the 2V of available headroom requires an LDO and a
>> rail-to-rail output op-amp.
>>
>>   
>>     
> Most rail to rail output amplifiers tend to have poor PSRR when the
> output approaches the rails.
> However with an EFC voltage swing of 20V the absolute noise and
> stability requirements may be relaxed somewhat but it depends on the OCXO.
> Having specifications for candidate OCXOs having -10V to +10V EFC swing
> would help in making a rational selection of components possible.
>   
>>> The brute force approach is to get 3 separate supplies. However, I
>>> don't think I need to go that far.
>>>
>>> Any ideas here are most welcome.
>>>     
>>>       
>> Do you have any idea what the external power supply will be?
>> It could be taken into account.
>>
>> It's noise coming OUT of the oven supply that matters, so subregulating
>> it doesn't help; muting the current spikes requires energy storage
>> (L and C).  Outside the pi filters, take the EFC supply, drop it
>> through an LDO, and watch the PSRR specs on the EFC components.
>>
>>   
>>     
> LDO's tend to be noisy and have poor high frequency line rejection.
>   
>> (Remember, the OCXO is not *that* sensitive to its own oven hash,
>> or it wouldn't work in the real world.)
>>
>> I often use the little things called "wound beads" or wideband
>> chokes", e.g. http://www.ctparts.com/widebandchokes.asp, for generic
>> supply filtering, but you may want a little more L in your pi filter.
>>
>>   
>>     
> A much higher inductance than can be achieved with a ferrite wideband
> choke is required as the 10544A oven switching frequency is relatively
> low being determined by a UJT oscillator.
> The appropriate filters and supply regulators for the 10544A are
> detailed on the datasheets which are available on Didier's site.
> The recommended circuit has much lower noise than any 3 terminal LDO
> regulator.
>
> Bruce
>
>