If you are on the lookout for a sub $30 Multimeter (or $15 on eBay) for working with low voltages, the best one right now in my opinion is the cute and tiny Aneng AN8008 TrueRMS Multimeter or the newer model, the Aneng AN8009 with NCV and temperature measurement. They have a lot of functions and can measure µA current, and both are a 9999-count meter. The continuity tester is reasonably fast, but there is one little problem that bothered me. It takes 5 to 10 seconds for it to settle on a correct voltage reading. When it finally ends, it’s spot on, but the waiting is annoying. But fortunately, there is a simple fix for this problem.
Fixing The Long Settling Time
After taking the meter apart, I started by measuring the input DC voltage to the meter. The measurement shows that the DC rail is quite noisy.
There is a decoupling cap on the input, a Rubycon 100 µF 16 Volt YXA capacitor, but when measuring it, the ESR is 1.4 Ω which is quite high. So first I replaced it with a low ESR Panasonic FM 100 µF 25V capacitor that measured 0.4 Ω instead.
Fortunately, there are three unpopulated 0805 sized pads on the board, marked C14, C15 and C16, so I soldered one 0.1 µF, 1 µF, and a 10 µF 0805 capacitors before I installed the 100 µF capacitor. Any run of the mill 0805 capacitor will do. But it doesn’t hurt to use a name-brand like Murata, Panasonic or similar. Here is a search for the required capacitors on Digikey.
The modification cuts down the noise on the power rail to one-third of what it was before the amendments! But wait, there’s more!
Now the peak to peak values where 8.5 milliVolt instead of 30 milliVolt, and there’s a lot less high-frequency noise. But what happens if we increase the capacitance even higher?
I installed a 1000 µF capacitor and knocked down the noise down to 3.6 milliVolts P-P! I tried a 3300 µF, but it was a diminishing return, so I decided to keep the Panasonic FM 1000 µF 10V capacitor installed. A noise cut down from 30 milliVolt to 3.6 milliVolt will undoubtedly help the ADC to give more ENOB. Here’s a search for the right type of 1000 µF capacitor on DigiKey.
Improve The Reference IC
The Aneg AN8008 uses a 1.2 Volt reference IC, the ICL8069DCZQ. It’s decoupled with a 0.1 µF capacitor, but when looking at the datasheet, they recommend a 4.7 µF capacitor. So I desoldered the small 0603 and stacked two 2.2 µF capacitors in its place because I didn’t have a 4.7 µF capacitor at hand. Same as before, any run of the mill 0603 or 0805 capacitors will do. But I prefer to buy name brands. The capacitor doesn’t have to be of high voltage rating; they are there just to filter out the noise to ground.
So Did It Make A Difference?
Yes! The meter settles on values immediately instead of 5 seconds of drifting back and forth. The reading is also much more stable over time because it used to wander around. It’s also spot on when compared to my HP 34401A 6 ½-digit multimeter.
So now a $25 multimeter performs even better. If you need a small portable multimeter (you can never have too many multimeters), this is a great buy! Don’t believe the CAT rating on it. I wouldn’t work with anything high voltage with this unit, but for low energy, it’s perfect. And after this modifications, it’s even better!