Why is measuring voltage with a DMM affecting my circuit? - Page 1
Forum section: electronics / beginners

author: dohnut
author_rank: Newbie
timestamp: June 21, 2014, 02:52:15 pm
content: I know there is probably a simple explanation, but I can't seem to find one.Here is what I'm doing.I've got a TMP36 (http://www.analog.com/static/imported-files/data_sheets/TMP35_36_37.pdf) (TO-92 package) connected to a PIC16F1824 (14-Pin DIP). The TMP36 is a temperature sensor with a voltage output. In the case of the TMP36, the voltage is offset by 0.5V but otherwise the ratio between voltage and temperature is linear. So 0.75V is (0.75V - 0.5V = 0.25V) = 25C and 0.8V = 30C. I have this running into the PIC which has the I/O pin configured as an analog input and the ADC module is enabled for the pin. The entire circuit is being powered at ~5.0V. I also have a 2-character LED module (serial interface) wired to the PIC show the temperature in degrees C. The LED module is being powered by the 5V rail and not the PIC.This is all working perfectly.I read the sensor and I am getting the correct value from the A/D for my ambient temperature. In this case my ambient temperature is 26C so the sensor is outputting ~0.76V. And then I'm doing a little math in the PIC and displaying the temperature on the LED module. If I put my fingers on the sensor, the temperature goes up. I take my fingers off, the temperature comes back down. Super.So here's the problem, when I probe with a meter the output from the temperature sensor is measured at ~0.61V and the PIC also sees this at the A/D which changes the temperature it is reading and displaying down to 10C-11C which would be expected if the output from the sensor was ~0.61V. If I take the probes off, the temperature display will jump back to 26C. So the meter appears to be causing a 0.15V drop in the output of the TMP36. FYI, to measure the voltage, I'm hooking one lead from the meter to ground and the other lead to pin 2 (voltage output) on the TMP36 and I also have the 5V power to the circuit bypassed with a 0.1uF cap per the reference diagram for the TMP36 in the datasheet.I have 2 meters, they both have the same affect. I actually measured the impedance of each meter with the other and they are both > 11M. So, I inserted a 10M resistor between ground and pin 2 on the TMP36 and there is no (measurable) affect on the reading as one would expect -- I still get a 26C reading.So why, with my meters having even higher impedance than the 10M resistor, are they changing the reading on the TMP36 when I measure it?Any insight would be greatly appreciated.

author: mcinque
author_rank: Unknown
timestamp: June 21, 2014, 09:41:49 pm
content: I'm definitely not a spark, but can this phenomenon be caused by the capacitance you introduce with the dmm probe cables on the PIC I/O port?

author: lapm
author_rank: Frequent Contributor
timestamp: June 21, 2014, 10:18:18 pm
content: My money would be on stray capacitance. The finger effect is classic sign of this.Datasheet actually offers hints of this:"Less than 50 ?A quiescent current""Capacitive Load Driving CL (No oscillations2) Min:1000 typical:10000 pF"All this comes to very very weak output driver stage. And honestly im suprised you get so close to real temperature readings under normal operating conditions... How ever the finger test shows there is room to improvement..Designing good pcb to minimize capasitive coupling is art from of its own usually. I know i could not do it without a lot more reading of subject...

author: dohnut
author_rank: Newbie
timestamp: June 21, 2014, 11:04:24 pm
content: Yes, I think you guys have the right idea.If I just take a 1.5ft piece of 22 gauge wire and stick it into the breadboard on pin 2 and leave the other end floating, the temperature reading drops by about 4 degrees C, so around 0.04V.I think the reason it is as accurate as it is, is because I have the sensor right next to the PIC. Even so it is probably about 0.5C off in all reality but that's close enough for what I'm doing right now -- which is really just playing around with some of these components.I've had similar issues trying to measure the current of very low-current LEDs. Putting the meter in-circuit affected the brightness of the LEDs pretty noticeably. I know there are other ways to measure current that minimize the impact on the circuit. It looks like measuring this voltage will require either shorter or more expensive probes. Honestly, it's not that important in this case, it was just bugging me, but it's good to know when you can trust your test equipment and when you cannot.I guess I've got a lot more to learn. I'll have to read up on that "capacitive load driving" parameter.Thanks for help.

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 21, 2014, 11:31:13 pm
content: Your 10M resistor substitution test was brilliant.You could do the same test with a capacitor and see if that causes the measured value to change.My presumption of the "finger test" was that touching the package actually increased the temperature.And hanging a long wire on the output pin may have the effect of a thermal "heat sink" as well as some extra capacitance.What if you take that same 22ga wire and wrap it around the package? That would be a "thermal" test vs. a "capacitance" test.

author: dohnut
author_rank: Newbie
timestamp: June 21, 2014, 11:55:09 pm
content: Interesting ideas Richard.To clarify the "finger test" -- which is a detail I probably should have left out. I was, in fact, just trying to warm the sensor with my fingers to get the reading on my LED module to change. I could warm the package gradually with my fingers applied and then with fingers removed, the package would cool back down to ambient gradually. It was just a test to make sure that the PIC was still doing its job since the ambient room temperature is relatively constant.When I see the temperature change with the wire or probes applied and then removed, the changes are immediate.However, I did try to touch all of the TMP36 leads simultaneously with my fingers and it did not seem to have any noticeable effect aside from warming the device and causing the temperature reading to increase.I'll play around with it some more, thanks for the ideas.

author: David_AVD
author_rank: Super Contributor
timestamp: June 22, 2014, 12:04:01 am
content: You have the recommended 0.1uF bypass capacitor on the V+ of the temp chip?

author: David Hess
author_rank: Super Contributor
timestamp: June 22, 2014, 09:53:10 pm
content: The multimeter lead is coupling noise into the output of the TMP36 which is rectifying it. You could add resistance at the probe tip or decouple the TMP36 output to ground.

author: babysitter
author_rank: Frequent Contributor
timestamp: June 23, 2014, 08:29:19 am
content: Series resistors, maybe 100k in front of the probes is what I vote for.Yes that is a voltage divider of 200k and about 10M, but at least you know it. O0A capacitor on the DVM input could be a good idea, too, but not necessarily.