Big project: M101v1-16 16-bit digital multimeter. - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: August 22, 2015, 03:18:04 pm
content: Feature specification:* True RMS meter* Built-in Li-ion rechargeable battery* Good accuracy* USB interface for datalogging and firmware update* Keypad and character screen interface* 4-wire resistor measurement only* Low-voltage ohm range (not common to see now)Schematics attached. If there is any issue lurking please tell me.

author: mariush
author_rank: Super Contributor
timestamp: August 22, 2015, 03:40:17 pm
content: Oh man, you really should use something better than mc33063 / mc34063 for +/- 15vCareful with IC105 (1117-33), depending on who makes it, it may not like ceramic capacitors due to very low esr. There's better options out there.I didn't check all the other pages so not sure if you did it but i mention it anyway... hope you isolate the usb from the meter in some way, usb is grounded to earth in computers and various devices. Wouldn't want some ground loops or to burn my computer when measuring some high voltage with the meter.I guess you could use some generic cheap data isolators to pass data safely to the CH340 chip and then the chip can stay connected directly to usb, if you don't need high speeds (like 1mbps+ or something like that)For usb power isolation you could use chips like ADUM5000 but these are limited to 100mA and they're expensive and inefficient (around 33%) or you could use an isolated psu which is more efficient, cheaper but takes up a lot more volume.

author: technix
author_rank: Super Contributor
timestamp: August 22, 2015, 04:27:37 pm
content: Quote from: mariush on August 22, 2015, 03:40:17 pmMy inputs are floating in the first place... After the MUX voodoo is an instrument amplifier.

author: codeboy2k
author_rank: Super Contributor
timestamp: August 24, 2015, 03:24:36 am
content: The in-amp following the mux is fine, it gives you floating inputs but not isolated from the USB ground. You'll need to be extra careful laying it out on the pcb and try to keep USB ground noise and battery charging currents localized. Your in-amp is still referenced to ground and so is the ADC, so if GND is bouncing because of USB activities, your ADC will see it. So keep that in mind, you may still want to isolate your USB, but that will introduce additional complexity since you want USB power to cross the isolation boundary to charge the battery. If I was doing this, I'd put the USB port, the charger circuit, the battery, battery monitor and +15V boost converter on the outguard side. The inguard side will have a transformer coupled off the 15V boost inductor, and get that across the boundary. USB data can be coupled across as well. Then on the in-guard side use a 15V invertor to get -15V from +15V, such as the ICL7662basically, echoing what mariush says re: mc34063. These are old and hard to work with. They do not do PWM control and use a weird switching pattern that causes sub-harmonics of the switching frequency. These are not expected and they bite you. I would say it is better to use a modern switcher that works up in the 1.5 to 2 MHz range. Then you can reduce the size of the inductor down to about 3.3 to 5 uH or so. With a high switching frequency you can follow it with an RC low-pass filter with a moderate 10 KHz corner frequency. The higher switching frequency allows you to use a smaller RC here too, so you can use something like 1 Ohm and 15uF, and only burn up .25W at 500 mA. However, the ADC uses less than 20 mA, so you could even go to 10 ohm, 1.5 uF and only burn 4 mW here. The higher switching frequency also allows you to use a ferrite bead followed by some C's for a LC filter that can remove the switching harmonics at the ADC's AVDD and DVDD pins. This will give you even greater PSRR here. You can also use the same ferrite bead + C's combination at the input to the 5V reference IC. Right now it's being fed from a noisy MC34063 that's hard to filter out without using large inductors and/or large capacitors.Instead of the LT1117-5 I would use the LT3088. Its about the same price, maybe cheaper even, but it has much better PSRR and an order of magnitude lower noise than the LT1117. It actually has a PSRR peak at 1.5MHz up to 60dB , whereas the LT1117 has less than 40dB above 100 KHz. So the LT3088 would be better matched to a high-speed switcher.re AD7610:1) the datasheet says it needs a 22uF decoupling cap at the REF input.2) the datasheet says it wants at least 10uF + 100nF decoupling caps on each of AVDD and DVDD.3) the datasheet says it wants the same 10uF + 100nF decoupling caps on OVDD (VDDIO on your schematic). You have the 10uF only.4) the datasheet says it wants the same 10uF + 100nF decoupling caps on VCC and VEE ( the +/- 15V supply)5) the datasheet says it wants a 10 ohm resistor between AVDD and DVDD if you use the same supply for both. This forms an RC filter with the 10 uF decoupling capThe mux has no AT-REF input. Do you not want to measure your own zero reference, i.e. auto-zero calibration for compensating offset before a measurement ?The mux has an input for the 6.9V LM399, but I don't see it being used anywhere else. What is the purpose of the LM399 if you already have the 5V reference going everywhere?For your ohms measurement, you've got the same 5 ranges as your voltage measurement. So, for the 4-wire current source (the Force leads) do you really need a 10-bit DAC with 244 uV resolution? you can just as easily select one of 5 fixed currents instead, and it doesn't even need to be accurately known, just temperature stable. Once the voltage measurement is soft-calibrated, you can soft-calibrate the resistance measurement too. Maybe in reality it's cheaper to use a $8.00 DAC and a Vref you already have then to try to get temperature stable resistors. I understand this could be the case here. But you can do it with an 8 bit DAC then. The TI DAC081S101 is $2.50 and has a tempco of 0.7 ppm/C. You'll want your 10 ohm current sense resistor R417 to be a low tempco, high wattage (5W?) resistor for lowest self-heating effects so as not to cause the current forced through the resistor to change during the reading. You want low tempco so that once it's calibrated it stays that way. Low tempco also helps with the self-heating problem. Also you will want to add a decoupling cap (10uF + 100nF) to the VCC and VREF pins of the DAC too.That's all I can see right now. In general I noticed many of the IC's are missing decoupling caps. That won't fly, you'll absolutely need them.Cheers! It looks like a nice project!

author: technix
author_rank: Super Contributor
timestamp: August 25, 2015, 04:55:02 am
content: Quote from: codeboy2k on August 24, 2015, 03:24:36 amRegarding the AD7610 decoupling, the reason I am not adding those 100nF decoupling caps is because people tend to assume that those 10µF caps are electrolytic, but in fact my designs calls for MLCC already, eliminating the need of that 100nF ceramic cap. Also the ADR02B have problems with capacitive outputs so I am going to put it close to the AD7610 trying to eliminate the REF decoupling cap.AT-REF does not need to be muxed in, since the input amplifier is already a differential amplifier. (the three op amps forms a classic instrument amplifier, taking the difference between AT-REF and one of the AT-something tap)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2015, 08:46:44 am
content: The resistance part (current source) has very little protection against external voltages. I also seen not protection of the DUT from too much output voltage. The second point is the limitation to rather low resitances. The current can be changes, but only over a limited range, and with low accuracy at the lower end. Changing the current over sevaral decades usually works better with changing the resitors - changing the voltag may give a limited 1:10 range, hardly more.So this part likely needs a complete make-over. As protection is not that simple one should included this from the beginning here.

author: c4757p
author_rank: Super Contributor
timestamp: August 25, 2015, 08:54:06 am
content: 100nF ceramic capacitors have better high frequency performance than 10uF ceramic capacitors, not just than 10uF electrolytics.

author: Marco
author_rank: Super Contributor
timestamp: August 25, 2015, 09:02:54 am
content: Why use an inamp instead of isolated power supply and USB connection?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 25, 2015, 09:21:50 am
content: The instrumentation amplifier is more like a try to get is easy when doing 4 wire ohms measurements and maybe alow for a sloppy grounding. Though I would also prefer using a good ground layout and single ended operation for voltage and currents.The INA is no excuse for not isolating the interface.

author: Marco
author_rank: Super Contributor
timestamp: August 25, 2015, 08:03:58 pm
content: Even for a 4 wire measurement it's not truly necessary, current source on one side, closed loop controlled voltage source on the other to force one of the measurement wires to ground.