DIY DMM: analog section design validation - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: May 30, 2015, 10:52:38 am
content: This is what I have in mind for the DIY DMM range switching mechanism. The DMM is designed to have 4.5 to 5.5 digit resolution, be microcontroller-controlled with autorange, and is powered from a few Li-ion batteries with built-in USB-based charger.Internal power rails: +/-5V, "3.3V" which is actually 3.6V directly from the batteries, and +12V for internal LM399 calibration target that is not always on.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2015, 11:24:59 am
content: This is not at all a useful circuit: The initial OP sets the input range, which is rather small with a TL084 powered from +-5 V. With the already amplified signal, there is no need to use a relay.The way to go, is to have the relay (well possibly 2 or 3) switch ranges before amplification, and than use something like CMOS switches (or JFETs) to do another 1 / 10 switching of the amplification. Have a look at a few circuits of commercial DMMs to learn from them. With likely a integrated ADC, the full scale voltage of the ADC is rather small. So not much amplification (hardly a factor of 10) is needed, but more a controlled divider. If the ADC has sufficient resolution, there is no need to do very fine gain adjustment with a digital pot.Likely some kind offset measurement is also needed, to corrects offsets of the amplifiers, especially if a resolution of better than 100 µV is planed.

author: Rerouter
author_rank: Super Contributor
timestamp: May 30, 2015, 11:44:56 am
content: Now what could possibly go wrong through exposing an op amp input terminal to the outside world i wonder?most use a low side switch for each resistor of the divider, which means that when the unit is not powered, the ADC sees the maximum division, and thus the lowest voltage,you then size your divisions so that the ADC never exceeds say 80% of its valid input, so you have headroom for dtecting the autorange, and a nice gap before some form of protection device comes into play,

author: technix
author_rank: Super Contributor
timestamp: May 30, 2015, 12:44:55 pm
content: Quote from: Kleinstein on May 30, 2015, 11:24:59 amThis is just range switching which is after the input selection portion of the circuitry. The input selection have all the protection, shunts and high voltage dividers. The leftmost op amp is the output buffer of that stage.The second op amp provides a selectable fixed 1 or 100 times gain with low leakage operation to allow 100uV or the like, so the d-pot, which is a resistor array with CMOS switching in reality, doesn't have to be a low leakage one.The d-pot in the feedback line (wrongly drawn actually) of the third op amp is the actual range switching mechanism. This allows me to set up a mathematical relationship between d-pot position (including the GPIO pin which controls the low leakage preamplifier/buffer), the ADC value and the actual voltage right after the protection mechanism. The high voltage divider would also be controlled by another GPIO pin through a relay.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2015, 01:27:03 pm
content: So the interesting part is still missing, and the part shown is more or less useless.There is no need for neither a 100 fold amplification nor a fine tuned amplification. Only a display oriented ADC like icl7135 might need fine tuning of amplification, but then this would be more like 256 steps for a 1-5% range. Though not really needed the inverting stage with a d-pot might be an option - though limited in voltage range and only a few steps of amplification are useful (e.g. 0.1 0.5 and may be 5 fold).

author: Marco
author_rank: Super Contributor
timestamp: May 30, 2015, 03:08:15 pm
content: Quote from: Kleinstein on May 30, 2015, 01:27:03 pmFor a low precision DMM you could decide to simply always leave the high voltage divider switched in. I agree with the rest though ... and if he really wants to make another boring old ICL7135 meter he can just trim it with normal pots.Doing range switching with a digital pot seems non cost effective, you don't need that many different settings ... a MUX with resistors can do it cheaper.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2015, 04:24:54 pm
content: Die resistors in some digital pot are surpassingly well matched and prices are still moderate. So they can be an alternative to a divider and CMOS switches. Something like MCP42010 has typical 1ppm/K TK matching at about $2 - however only up to 5 Volt. The AD516x still offers 5ppm/K matching at $3-4 and go up to 15 volts. MCP41HV31 works up to 36 V with still typically 15 ppm/K matching in the $1-2 range. Having a similar quality divider like 1:10 steps by hand is not that much cheaper. A good way to do it by hand would use something like 6 identical (preferable thin film) resistors, dividing 3 in parallel against 3 in series - this way chances are good to have good TK (and aging) matching even with 50 ppm/K resistors.To have the divider always on is an option, but works best if the following stage works up to relatively high voltages, as the ratio can be smaller then. So even an amplifier with more that +- 15 V supply might be useful then.

author: technix
author_rank: Super Contributor
timestamp: May 30, 2015, 07:36:36 pm
content: Quote from: Kleinstein on May 30, 2015, 04:24:54 pmI do still have an GPIO pin from the AD5280 d-pot chip so if I want to use a relay to switch the divider out I sure can (it is a dual supply chip, and since it is used as a potentiometer only the relative tempco matters) but the +/-5V supply voltage can be problematic for the op amps at high input voltages I may just leave the high voltage divider in there for the volts range and switch this 100x low leakage preamplifier in for a lower scale. AD5280 can be a lot cheaper than matched resistor arrays with similar relative tempco actually.

author: technix
author_rank: Super Contributor
timestamp: May 30, 2015, 07:40:02 pm
content: Quote from: Marco on May 30, 2015, 03:08:15 pmAlthough I did but a few ICL7135 (since it is from the same seller offering AD736, the RMS to DC converter, as my meter will be a true RMS one) but it is not going to be used unless as a last resort, and I will not use its internal reference (any ADC anyway, if possible) but an external one based on MC1403 (or ADR02B, or ADR03B, not decided yet) and another AD5280 d-pot, with LM399 as internal calibration target to allow automatic reference trimming.