Re: TMP421 on the preamp board

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Gerard and Yves,

Interesting indeed. I don't recall seeing this on the board I have but I 
will check it.

The TMP421 will be employed to just take temperature measurements on 
demand (measure and convert) and infrequently.(the TMP421 can be 
programmed for single shot or continuous operation and at various rates).

The TMP421 also has an internal sensor and can measure one remote 
sensor. The remote sensor is on the middle ASIC, as you point out, and 
is isolated from any circuitry on the ASIC, except for perhaps some 
capacitive coupling through the bulk Si. Mitch can tell us exactly what 
the arrangement is.

I completely agree that we should not have any situations that affect 
the operation of the ASIC in any form. I will test and let know what I 
find next week.

I also see great signals from Yves' log. But what exactly caused the 
oscillations in the first place? I have not seen any good explanations 
as for the cause so far but I am happy for the progress. Perhaps you and 
Yves could right a note on the findings.

Yves - you may want to try turning off any fluorescent bulbs you have in 
the lab for two reasons: the ballast transformers, if present, can be 
very noisy; and the UV from the bulbs may cause extraneous pulses on the 
straws through the gaps from the wrapping. I first observed this at BNL 
(E850) where we had aluminized mylar straws. The UV from the bulbs 
overhead, and which penetrated the straws through the gaps in the 
wrapping, would knock electrons out of the aluminum surface inside the 
straws and produce very noticeable pulses. The surface work function on 
your copperized Kapton straws is much higher so that this may not be a 
problem for you. Another interesting observation was that we could make 
the inside of the straws glow blue (plasma) in the dark at higher 
voltages, in the streamer regime. The blue color was perhaps because of 
the Argon (I believe we used Ar-ethane 80/20).

Thanks for the feedback. Regards,
Fernando



Gerard Visser wrote:
> Hi Fernando,
>     I encountered a mysterious effect this week when at CMU... There 
> were always spikes visible in the output of the preamp board, at least 
> I can say "always I saw them on most channels". I saw this on both the 
> boards (mine and CDC's). I thought it was external interference from 
> something over there. But we never could track down what, and 
> shielding never had much effect on this, though it did help with the 
> 88.3 MHz interference (dramatically now as I see from Yves!).
>     I left Yves with "my" preamp board (with the 59 Ohm resistors 
> installed) and returned here with "his". I set it up yesterday and was 
> amazed to find exactly the same spikes occurring, at roughly 100 kHz, 
> same as I saw at CMU.
>     So it had to be an internal problem; I tried powering from a 
> battery, still the same. We tried a Tek 465 scope at CMU, so I knew it 
> was not the digital scope generating interference.
>     Turns out, it was from the TMP421 circuit - this works by 
> switching (apparently rapidly) between two bias current levels in the 
> temperature sensing diode, no doubt at a frequency of about 100 kHz. 
> The actual problem on this board was a poor solder connection at the 
> bypass cap by the TMP421 chip. I resoldered and there is now _no_ 
> detectable noise spike. [Triggering the scope on the output noise and 
> averaging the trace I get a nice symmetric function, due to the 
> correlation of the noise - previously if you did this you got a highly 
> asymmetric function that looked just like the response of the ASIC to 
> a charge pulse input.]
>     I'm not writing to complain about soldering though - what I really 
> need to say is that the middle ASIC, the one where the temperature 
> diode sits, may still suffer some effects from the TMP421 circuit. I 
> can't test this because I don't have a middle ASIC on either of these 
> boards. But you do, right, so you can look into this...
>     I think that either
>     - It will work fine (generates no noise spikes on any channels), or
>     - It generates some noise spikes, but this can be remedied by 
> putting a few hundred pF to ground on the diode side of R144,R145 on 
> your schematic, or
>     - It's hopeless and we should reconsider how to use the internal 
> temperature diode, or should just use an external temperature sensor.
>     I think we should not have a situation where the board generates 
> noise spikes even at a low rate, unless it only happens a short while 
> in response to controls command to measure the temperature. It seems 
> that the TMP421 is continuously measuring the temperature regardless 
> of when it is read out. That is probably acceptable only if it can 
> generate no interference.
>     Sincerely,
>
>     Gerard
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