[time-nuts] GPS Disciplined TCXO

[Nick Sayer]

> On Oct 23, 2015, at 10:18 AM, Nick Sayer <nsayer at kfu.com> wrote:
> 
> 
>> On Oct 23, 2015, at 9:10 AM, Attila Kinali <attila at kinali.ch> wrote:
>> 
>> On Fri, 23 Oct 2015 07:30:10 -0700
>> Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>> 
>> 
>> 
>>>> 
>>>> In the AD5061 datasheet (Rev C) on page 3, at the bottom,
>>>> section "Reference Input/Output", item "V_REF Input Range".
>>>> Yes, it will not fry the device, but it will not work up to spec either
>>>> (unfortunately, manufacturers never mention what part of the spec gets
>>>> degraded if you violate something).
>>> 
>>> I do see that now. If you look at the footnote, figure 27 which it references 
>>> shows that the Vdd headroom with a VRef of 3.3 volts is more like 20 mV or 
>>> so. I would posit that the headroom issue is more likely to result in non-
>>> linearity at high DAC values, perhaps because the output amp may not be 
>>> capable of full rail-to-rail operation. 
>> 
>> It is not clear where the limit comes from. It could be the ouptut buffer,
>> which would then be a no-problem. Or it could be the input buffer for V_ref,
>> which then would lead to an un-buffered, and hence unstable internal reference
>> voltage. Or it could be a problem of one of the DAC internal stages, that
>> needs the headroom….
> 
> I’ll try asking Analog Devices and see if I can get an answer to the question of what happens when Vdd=Vref=3.3V. All I know is that in practice, it’s working as designed, including meeting the gross tuning slope predictions within 10%.

They got back to me quickly. They said that as you increase the DAC values, there will come a point where the output will stop rising as you get to the point where the headroom is necessary. That is, the top values will get clipped.

The argument could be made that an alternate configuration could result in reclaiming that lost precision, but for now, I think I’m going to let it go. The current configuration counts on the midpoint on the input being the same as the midpoint on the output. If you reduce Vref, that’s not true anymore. If that’s not true, then the tuning range is asymmetric, and that means you’re going to throw away part of it. I guess the headroom issue means I’m throwing away part of the top end asymmetrically as well, though. *shrug*