DIY through-hole multimeter project

[dardosordi]

Since it is a common suggestion to use some microcontroller involvement, how about this idea:

* Emit the 100kHz reference clock from the microcontroller
* Use the microcontroller to switch ranges
* Use DACs and digital pots to calibrate the circuit digitally

Delta-Sigma ADC is still out of the scope as I still don't want to touch SMT on this design.

About the choice of the microcontroller, which one seemed better? I am relaxing the MCU selection from AT89C2051-only:

* AT89C2051-24PI @ 24MHz (12T 8051 architecture)
* IAP15F2K61S2-35I-SKDIP28 @ 30MHz (1T 8051 architecture, built-in SPI master hardware for DAC)
* IAP15W4K61S4-30I-PDIP40 @ 30MHz
* ATmega328P-20PU @ 16MHz (AVR - Arduino, we all know it)
* PIC18F45K20-I/P @ 64MHz (3.3V PIC)
* PIC18F4550-I/P @ 64MHz (3.3V PIC with USB)

If a 40-pin MCU is used, maybe I will also move the display decoding and driving into the MCU, and simplify the display board into a MAX7219-driven one, or a HD44780-based display module.

What's your goal? If you are building this just for you then go ahead with the one you have available or some code done. If you plan this as something a beginner may build go for the arduino.

[dardosordi]

AFAIK there is a way to read the ICL7135 by reading the sign, a sync signal and let the µC count pulses instead of reading the BCD values. This needs less pins on the µC. Still there is not that much room for interpretation for the µC on the ADC data. At most may be scaling with 2 to allow a +-4 V range (last digit always even).

One advantage of using an SD converter and µC for scaling would be that one is not bound to the 19999 count ranges. So one could have ranges to maybe 4 V or so which would be about the limits for an ICL7650 amplifier (one of the few available AZ OPs in DIP) for the input.

I don't know sigma delta converters in THT case. But there are a few in SMT that are still relatively easy to solder. Something like SOT23-6 or SO-8 are still relatively easy and may go to a small adapter board.

You can read the output counting the pulses and there are outputs to signal over and under range. You probably won't need those if using a micro, though.

[dardosordi]

The internal ADC would be more for auto-ranging, e.g. measure the approximate positive and negative peak voltages to detect possible clipping. So no high accuracy needed, but 2 or 3 input would be nice. Comparators would also work - but an ADC might be easier.

You can autorange using just the ADC output.

[David Hess]

The internal ADC would be more for auto-ranging, e.g. measure the approximate positive and negative peak voltages to detect possible clipping. So no high accuracy needed, but 2 or 3 input would be nice. Comparators would also work - but an ADC might be easier.

You can autorange using just the ADC output.

I could see using the crummy microcontroller ADC to sample the input allowing for fast autoranging.  The ICL7135 is pretty slow.

[technix]

The internal ADC would be more for auto-ranging, e.g. measure the approximate positive and negative peak voltages to detect possible clipping. So no high accuracy needed, but 2 or 3 input would be nice. Comparators would also work - but an ADC might be easier.

You can autorange using just the ADC output.

I could see using the crummy microcontroller ADC to sample the input allowing for fast autoranging.  The ICL7135 is pretty slow.

Hmm that would be interesting. The frontend, MCU and autoranging logic being one module, and ICL7135 and display being another. (I am stuck in two-board construct anyway)

Range switching have to be done on a separate MCU on the display board though. The board-to-baord connection can be 30cm of Cat6 and a 4-pin molex connector: two twisted pairs for RS485 between the two MCUs (generate as little interference as possible) one twisted pair for differential input signal of the ADC, and use the Molex for power (DVCC, GND, AVCC and AVEE.) This gives me a common autoranging frontend that guarantees a 2V FS output, and allows me to try different ADC for the display and output.

[Kleinstein]

With an MCU there will be not much (if any) logic chips for auto-ranging it is more like quite some CMOS switches and a few relays (e.g. to switch the high voltage dividers and maybe the ohms current source). I would prefer CMOS switches over relays if one has the choice.

Auto-ranging just on the ADC output could be tricky, as the average DC output can be still in range, but short time there might be clipping. So one should also have some kind of detection for clipping, e.g. be comparators or maybe diode / capacitor circuits and the ADC to check from time to time. The ADC ICL7135 should have quite some headroom (e.g. up to 3.5 V) - but the input amplifier might not have that much in some ranges.
Using the ADC could also check for the limits on the next lower range - so no need to actually switch to higher gain, just to find out that this would cause an over-range due to short time excursions. It somewhat depends on the input stage how much reserve is there beyond the nominal range.
It is a little the question if one needs analog peak detection circuits or can use the ADC to just sample the signal relatively fast.

[David Hess]

Auto-ranging just on the ADC output could be tricky, as the average DC output can be still in range, but short time there might be clipping. So one should also have some kind of detection for clipping, e.g. be comparators or maybe diode / capacitor circuits and the ADC to check from time to time. The ADC ICL7135 should have quite some headroom (e.g. up to 3.5 V) - but the input amplifier might not have that much in some ranges.

The ICL7135 has overrange and underrange outputs but since it typically runs at 3 readings per second, I would at least consider using the MCU ADC for this.

I have never seen a digital meter with an integrated ADC bother with crest factor detection.  The ICL7135 does not have much input range at maybe +/- 4 volts but the input RC filter shown in the examples has a cutoff of 16Hz making this almost a nonissue.

Using the ADC could also check for the limits on the next lower range - so no need to actually switch to higher gain, just to find out that this would cause an over-range due to short time excursions. It somewhat depends on the input stage how much reserve is there beyond the nominal range.
It is a little the question if one needs analog peak detection circuits or can use the ADC to just sample the signal relatively fast.

I think my old Tektronix handheld meter does crest factor detection but it is unusual in not using an integrating ADC; apparently it does RMS measurement digitally implying some type of sampling converter.  After one lawsuit, Fluke bought this series of meters from Tektronix and made an agreement that Tektronix say out of the market for handheld meters.  I do not know if Fluke continued to make them or just discontinued them which is too bad since I really like their performance and features.