Specs for Precision Gold Whitegold WG 023 Multimeter - Intersil 7129 chip

[KirstyA]

I have a Precision Gold Whitegold WG 023 multimeter but I don't have the manual for it so don't know the spec.
Does anybody have the little booklet that listed the spec??

I have found the spec for the WG 020/021/025/026 but those are 3 1/2 digit meters with a DC Volts accuracy of 0.5%
The WG 023 is a 4 1/2 digit meter with an Intersil 7129 A/D chip.
When I looked up the spec for this chip I was shocked to find it's best accuracy is 0.005% !!
I doubt this meter is anywhere near that good, but if anybody has one and has the booklet that came with it or know where I can get the spec from I would be very grateful.

[Kleinstein]

The 0.005% numberis just the resolution of 4.5 digits, and maybe the INL error the ADC chip may reach. Dual slope ADCs can be quite good an 4.5 digits is possible with no big difficulty.  However this is just the ADC. Much of the uncertanty comes from the reference and the divider at the input.
From the pricture is looks like the CAT rating is more like the Chinese fake type and based on wishful thinking / best case and not real tests. The connector for the Hfe test is at least a bit better than typical, but still likely not really compatible with a CAT 2 rating.

[AVGresponding]

The 1996/7 Maplins catalogue lists the basic DC accuracy as 0.5%

[AVGresponding]

The 0.005% numberis just the resolution of 4.5 digits, and maybe the INL error the ADC chip may reach. Dual slope ADCs can be quite good an 4.5 digits is possible with no big difficulty.  However this is just the ADC. Much of the uncertanty comes from the reference and the divider at the input.
From the pricture is looks like the CAT rating is more like the Chinese fake type and based on wishful thinking / best case and not real tests. The connector for the Hfe test is at least a bit better than typical, but still likely not really compatible with a CAT 2 rating.

I suspect it does meet its safety rating, though perhaps marginally. It was not unusual to see this brand in various teaching institutions in the UK in the 1990s, and at the time Maplins was quite a large retail business. I think flagrant fraud in this area would not have flown.

They came to grief by trying to become a PC component supplier, pretty much ignoring their existing customer base. A shame, they were a good source of some components in the days before you could get stuff online, and the likes of Farnell and RS "required" you to have a trade account.

[KirstyA]

Thank you so much both Kleinstein and AVGresponding for such detailed and quick responses. 
This is my first post on this forum and it's nice to see such friendly and useful replies. 

I take it that you have all the old Maplin catalogues AVGresponding?

Looking more closely at the specs of the WG023 and comparing them with the WG025 I don't think the 0.5% DC Volts accuracy can be right for the WG023.
The WG025, which is 3 1/2 digits and a cheaper meter, claims 0.5% +1 digit on DC Volts, whereas the more expensive 4 1/2 digit WG023 says 0.5% +4 digits.
That makes it worse than the cheaper WG025. What's the point of the extra digit even?

Also the WG023 claims best accuracy of 0.2% +2 digits for the Ohms ranges, whereas the WG025 is 0.8% +2 digits, which is what you would expect from a 0.5% meter.
How can the Ohms ranges be better than the DC Volts ranges when it would be standard practice to use one of the DCV ranges with a known current source to measure the voltage across the resistor??

My gut feeling says this meter is more likely to be 0.1% on DC Volts. That would be more realistic for the 0.2% on the Ohms ranges too.
I'll find out when I come to calibrate it.

[Kleinstein]

The ohms ranges are relative to a reference resistor. In the ohms mode the voltage reference should not have a significant effect.
In the votlage mode the voltage reference can be an important part of the uncertainty - so this may very well be the limiting factor.

For the coparisome +-5 digits is not the same for a 3.5 and 4.5 digit meter. The digit steps on the 4.5 digit meter are smaller so 0.5% plus 1 digit for 3.5 digit resolution would be equivalent to 0.5% plus 10 digits with 4.5. digit resolution. So the WG025 is still a littel higher accurac, though not by very much.

The spec limits are the kind of worst case errors and most meters would be somewhat better than the specs to make sure they are still OK under more extreme conditions; like higher temperature or after a few years.

Unless one has a much better source / meter, I would not change the calibration / adjustment trimmers. The usual rule of thumb is to use a DMM that is about 10 x better than the meter to calibrate.

[KirstyA]

The ohms ranges are relative to a reference resistor. In the ohms mode the voltage reference should not have a significant effect.
In the votlage mode the voltage reference can be an important part of the uncertainty - so this may very well be the limiting factor.

I don't think I have ever known a meter to have better accuracy on its Ohms ranges than it's DCV ranges.
I see what you are saying about a known reference resistor and doing a comparison against the unknown, but I thought all/most meters adopted the approach of passing an accurate known current through the unknown resistor and measuring the voltage on a DCV range? Or is this only done on some more expensive meters?

For the coparisome +-5 digits is not the same for a 3.5 and 4.5 digit meter. The digit steps on the 4.5 digit meter are smaller so 0.5% plus 1 digit for 3.5 digit resolution would be equivalent to 0.5% plus 10 digits with 4.5. digit resolution. So the WG025 is still a littel higher accurac, though not by very much.

Yes you are correct. I hadn't given this enough thought before posting this point.

Unless one has a much better source / meter, I would not change the calibration / adjustment trimmers. The usual rule of thumb is to use a DMM that is about 10 x better than the meter to calibrate.

I do have 2 Solartron 7150's and a Datron 1065, all needing attention/repairs, (eBay purchases), which most probably have all lost their calibration data, so i'm no better off ATM with no accurate reference sources, lol.

I am currently re-building up my electronics lab again after many years, so am at the stage of cleaning everything up, repairing where needed, then will calibrate everything, hence why I wanted to obtain the specs of this meter when I come to calibrate it.
When everything is repaired i'm likely going to hire a multi function calibrator or a high precision lab standard multimeter for a week and calibrate all my equipment with it.
I hope at this stage to have the money to have built my own voltage reference and precision resistance box which I will also characterise against the hired lab standards.
The eventual goal is to have my own lab standards with which to calibrate all my equipment against, requiring the hire of expensive lab standards equipment very rarely, (say every 2 years), as a reference check.
Over the years I will then be able to plot my uncertainty budget to a reasonable degree, good enough for what I need.

From the pricture is looks like the CAT rating is more like the Chinese fake type and based on wishful thinking / best case and not real tests. The connector for the Hfe test is at least a bit better than typical, but still likely not really compatible with a CAT 2 rating.

This meter, and a second one I just acquired off eBay, (not sure why I bought a second one other than sentimental value of having the first one so long and the fact it's got real components in it I can repair, lol), are going to be relegated to just Breadboard Development/Design only, i.e. low voltages and low currents. For this they will be perfect, especially the ultra sensitive DC Current ranges. So i'm not worried about the CAT ratings at all.

[KirstyA]

They came to grief by trying to become a PC component supplier, pretty much ignoring their existing customer base. A shame, they were a good source of some components in the days before you could get stuff online, and the likes of Farnell and RS "required" you to have a trade account.[/color][/size][/b]

Maplin also started selling consumer junk like radio controlled toys and other stuff at elevated prices, a big part of their downfall, same way Tandy went, and now you have BangGood as the king of consumer junk.
My GoTo's these days are Farnell and Mouser, with RS if I have to 

[Kleinstein]

The usual way for the Ohms measurement is to send a fixed (for the range) current through the resistor. However the current is derived from a resistor and the reference voltage. So the value of the reference voltage does not enter in the result. There is still a bit more of the constant current source to enter the result.
There is alternative version with the same current flowing thrugh the DUT and reference resistor and than measuring the voltage over both resistors and do the math to get the resistance. This is e.g. done in the Keithly 19x meters.

The ICL7129 and similar ADC chips have a high impedance differential reference input. A possible implementation here is to have the reference resistor directly at the reference and this way avoid most of the uncertainties from the constant current source.

It is unusual to have power accuracy in DC volts more than with resistance, but quite possible, if the reference chip is relatively low grade. The 0.5% accuracy is a bit disappointing for a 4.5 digit meter. 0.2 % for resistance is already quite good. So it is a bit unusual combination. More moden 4.5 digit meters tend to have better accuracy in the volts mode.

[AVGresponding]

I managed to whittle it down to just a couple of catalogues, the 96/7 one and the 2000 one...   
Also a Farnell 95/6, a full set of 98 RS, and a partial 95/6 RS. These days most information can be found on the web but they still come in handy now and again.

Proceeding on the basis of a probable typo in the earlier catalogue I dug out the Y2K one (found a 256GB SSD I'd been looking for and a 1TB spinning rust drive BNIB that I forgot I had while getting the catalogues out, thanks!), and though it still states 0.5% in the blurb, the tables show a different and more plausible story of 0.05% DCV basic accuracy.



I almost always use Rapid these days, though I have accounts with all the big players and quite a few small ones, you never know when you might find a typo in the pricing that gets you a lifetime's supply of some useful component...

[Gyro]

Ha, I still have the WG022 in the next column - care to shift over a bit?

[AVGresponding]

Ha, I still have the WG022 in the next column - care to shift over a bit?

I should probably not put the catalogue away again for a while...   

[Gyro]

Thank you. Yes, slam it shut while you're ahead. 

[KirstyA]

Proceeding on the basis of a probable typo in the earlier catalogue I dug out the Y2K one (found a 256GB SSD I'd been looking for and a 1TB spinning rust drive BNIB that I forgot I had while getting the catalogues out, thanks!), and though it still states 0.5% in the blurb, the tables show a different and more plausible story of 0.05% DCV basic accuracy.

I found this meter based on the same Intersil chip. It too has an accuracy of 0.05% on 200mV DC. The circuit looks very similar to what's in the WG023, with the TL062 op amps ect. if you ignore the capacitance function which the WG023 doesn't have.
I suspect they have just cloned it and added the capacitance function. Further verification that the WG023 is 0.05% on DC Volts.

Uni-T UT56 schematic

[Kleinstein]

The circuit for the WG023 and similar meter with the ICL7129 chips usually follow the intended circuit for the chip. As the ADC chip contains much of the circuit, including the input amplifier, there is not that much to change. The accuracy still mainly depends on the input divider and reference quality. So essentially the same circuit digram can lead to 0.05% accuray or 1% uncertainty, just by using cheaper parts.

The TL062 OPs are typical low cost, low power OPs of the time (and still popular). They are used for the AC part and capacitance part. So they would not effect the DC accuracy.

[KirstyA]

The usual way for the Ohms measurement is to send a fixed (for the range) current through the resistor. However the current is derived from a resistor and the reference voltage. So the value of the reference voltage does not enter in the result. There is still a bit more of the constant current source to enter the result.
There is alternative version with the same current flowing thrugh the DUT and reference resistor and than measuring the voltage over both resistors and do the math to get the resistance. This is e.g. done in the Keithly 19x meters.

The ICL7129 and similar ADC chips have a high impedance differential reference input. A possible implementation here is to have the reference resistor directly at the reference and this way avoid most of the uncertainties from the constant current source.

It is unusual to have power accuracy in DC volts more than with resistance, but quite possible, if the reference chip is relatively low grade. The 0.5% accuracy is a bit disappointing for a 4.5 digit meter. 0.2 % for resistance is already quite good. So it is a bit unusual combination. More moden 4.5 digit meters tend to have better accuracy in the volts mode.

You have given me some useful theory to digest here, thank you. I now find I want to look for articles on these techniques. I'm always keen to learn new things.

Further investigation from AVGresponding's newer Maplin catalogue and information I found out about a similar meter, Uni-T U56, (see DCV spec and schematic), would indicate it is actually 0.05% on DC Volts, so not such a disappointment after all.

[KirstyA]

The circuit for the WG023 and similar meter with the ICL7129 chips usually follow the intended circuit for the chip. As the ADC chip contains much of the circuit, including the input amplifier, there is not that much to change. The accuracy still mainly depends on the input divider and reference quality. So essentially the same circuit digram can lead to 0.05% accuray or 1% uncertainty, just by using cheaper parts.

The TL062 OPs are typical low cost, low power OPs of the time (and still popular). They are used for the AC part and capacitance part. So they would not effect the DC accuracy.

I'm inclined to think the WG023 is 0.05% over the majority, (if not all), the DCV ranges and that the Uni-T UT56 has used less accurate components in the divider, hence the less accurate specs on anything but the default A/D range.

[Kleinstein]

The datasheets for the ICL71xx ADCs are often quite good and definitely worth reading at least one of them (they are quite similar).

With the Datron 1065 waiting for repair, it could also be worth a look at the service manual for this meter (or one of the similar ones like 1061). The ADC pronciple is still similar.

[KirstyA]

The datasheets for the ICL71xx ADCs are often quite good and definitely worth reading at least one of them (they are quite similar).

With the Datron 1065 waiting for repair, it could also be worth a look at the service manual for this meter (or one of the similar ones like 1061). The ADC pronciple is still similar.

I have downloaded one datasheet for the ICL7129 so far, but i'll get one of the 7106's as well.
I have the service manual for the 1065 and the 1081, so i'll study those when I get time.

THANK YOU so much for your valuable input Kleinstein. It is much appreciated