TMP512 Temperature Errors (Local & Remote)

Hi Robert,

Thanks for using the TMP512. 

I'm a little confused about your questions, so to clarify:  Are you stating that the +/- 1C accuracy of the local temperature sensor on the TMP512 is not accurate enough for your application and you're trying to increase your accuracy, or are you stating that you are not achieving the +/- 1C accuracy of the local temperature sensor on the TMP512?

I look forward to your reply.

Regards,
Collin Wells
Linear Applications

Collin,

Figured out the problem with the temp reading difference… ( programming error) was reading it as a word (msb)(lsb) and performing the value conversion, but its 13 bit not 8/16 bit thus some bit shifting had to take place which corrected the issue with the Local temp and remote temps be so drastically different.  That is fixed now.

The difficulty I’m dealing with now is the amount of temp DRIFT that is occurring with the remote sensors.  

Using 18 gauge, 2 conductor shielded (shield not connected to the 2n2906 except thru caps), with 2n3906 on the ends of each of the remote sensors here on the bench.

I’ve tried various sizes of caps parallel to the sensor and from the sensor to ground with no improvements UNLESS, I disconnect the Ground (shield) of the sensor leads then the temp settles down.

Unfortunately the grounds have to be there because of the HIGH RF exposure inside the RF Amplifier Tube chambers (without the shields in place the temp sensors and values go absolutely nuts -worthless readings).  With these ground shields in place there is no disturbance to the temp (as far as I can tell) but the problem is that the temp never settles down, its constantly changing +/- 4 deg C (tenths would be acceptable, but full degrees is not) so I'm trying to find a way to get this filtering under control.

We have been using the TMP175 for this application and they work great, but we wanted to consider this TMP512 for the fact it can be mounted in a remote location inside the amp away from the HIGH RF EXPOSURE area and let us monitor each TUBE's temp independantly, where as with the TMP175 we monitored only 1-tube.  (there are 3 chambers were trying to monitor temps in).  Thus this would be great for this application if I can get it to settle down.

Other alternative we've considered is running independant "Addressable" TMP175 one at each of the chamber locations, which is still out for debate right now due to the size issue requirements of the PCB for the mounting of the TMP175 SOIC8, Ugh oh the delima's... Coffee time!

Open to suggestion if you have some.

Thanks

73 Rob

K0RU

Collin,

Thanks for the study report, after looking through it with out more details I would say using the Shield Grounded was the more quieter solution.  It also appears to be the case here in my proto-type.

Currently I've connected the 2n3906 Grounded Collector Transistor mode and it appears to be working pretty well.  No interference from the 2 kW amplifier on the bench at full load, the temperature varation is+/- 5 tenths pretty acceptable considering the layout.   Required the following to achieve this

2n3906 (grounded collector tied to shielded conductors shield), parrallel 680pf caps between DXP & DXN @ the transistor on the sensor end.

Temperature is +/- 5 degrees of Local (no n-factor initialized yet) with a drift of about 1-2 tenths of a degree every few seconds, with up to 5 tenths of a degree every min or so.

I'm setting up a SMT MMBT3906 right now with some SMD 680pf caps and then tie that to the sensing lead and let it ride for awhile and see how it plays.  I'll see if I can get the scope on it as well to do some recordings.

Thanks for your input though

Rob - K0RU 73