Electronics > Metrology

REF70 new from TI

<< < (10/10)

Andreas:
Hello,

for a LS8-package the change of your sample is rather high.
Eventually there is some influence from the (EPOXY) PCB where it is mounted.
(which humidity do you have in your area?)

compared to the LTC6655 its even worse than my first measurement of a LTC6655 in MSOP package (ADC14)
The pure LTC6655BLS8 (without influence from the PCB) I measured with +/-4 ppm hysteresis.
see:

https://www.eevblog.com/forum/metrology/ltc6655b-long-term-drift/msg2355873/#msg2355873

with best regards

Andreas

exe:
The datasheet suggests that consecutive thermal cycling should have smaller impact on the voltage. Still, 56ppm shift is more than I'd expect.

I'm fighting my greediness and I'm willing to experiment on my max6226 for the sake of fun science. Stay tuned :).

chuckb:
My lab has a nice variable speed HVAC system with some humidity control. It is typically 23C +-0.2C. I have a number of circulation fans running so the temp control has been verified in the different corners of the lab. Most of the time it has a 40% RH +-2%. The last few days have been cool and raining so the humidity was 43%.

I have the REF70 demo PCB in a small heated chamber. For this test I use a SciGene Hybex Incubator ($50 'bay) with some 1/4" steel plates in the bottom to add thermal mass. There is no humidity seal on the chamber. I have not noticed any electrical interference between the heater controller and the DUT on previous sensitive electronics tests.

Attached is the plot from the last several days of testing. The chip temperature was monitored with a small, stick-on, OMEGA DIN 100 Pt Class B RTD. Datasheet attached. The resistance was converted to temperature by a 4-wire connected K2001 set for D100 RTD. A Fluke Model 52 with a thermocouple crosschecked the temperature. The two sensors agreed within 1 deg C.

Setup -
5V applied to the REF7025 demo PCB. The output voltage was monitored with a Keithley 2002.

Procedure -
I would change to a new temperature, wait about 2-3 hours, verify that the system had stable temp and voltage, and take a reading. The DUT was suspended in the air inside the chamber.

The testing started in the lower left corner of the graph at 2.4996V. As I raised the chamber temperature the output voltage decreased at 2ppm / deg C. I stopped testing at 61 deg C. I let it cool over night and the voltage had increased to 2.4997V.

As I increased the temperature in steps on the second day the voltage dropped in a similar manor of about -2ppm /C.

This time I let the PCB soak at 61C for 23 hours. The voltage increased quickly at first then it ended up with a +150uV total shift. The first 100uV of movement happened in 4 hours.
Over the next 8 hours I tested the chip as it cooled. With 1kg of steel in the chamber it does not cool off very quickly. It ended up 200uV above where it started 36 hours earlier.

Later, I will do a few more temperature cycles to 61C then a few going up to just 30C.

The first large hysteresis step is within the REF70 data sheet limits for a 0-70C temperature change. However the factory expects the voltage shift to reduce with cycles. I have not seen that yet. Linear Tech recommends stress relieving cuts in the PCB for their SMD Voltage references. That may help.

Navigation

[0] Message Index

[*] Previous page