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.

Rigol oscilloscope trace of the AN8008’s DC rail before the modification, cursors marking 30.3 mV of peak-to-peak noise

A DER EE DE-5000 LCR meter reading the original decoupling capacitor at 104.15 uF with an ESR of 1.4 ohmThe noise spikes are over 30 millivolts from two AAA batteries! There’s something adding noise on the DC rail.

Decoupling Capacitors

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!

The opened AN8008 board with the replacement low-ESR capacitor arrowed and the empty C15 and C16 decoupling pads circled

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?

The same scope trace after the decoupling capacitors went in, peak-to-peak noise down to 8.96 mV

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 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.

Scope trace with the 1000 uF capacitor fitted, peak-to-peak noise down to 3.92 mV

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.

The board with a red arrow marking the 0.1 uF capacitor beside the ICL8069 reference, labelled to be replaced with 4.7 uF

 The finished modification: a 1000 uF 10 V electrolytic capacitor wired in beside the AAA battery holder

So Did It Make A Difference?

Yes! The meter settles on values instantly, 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.

The modified AN8008 reading 10.00 V, held in front of an HP 34401A showing 10.00169 V DC

The AN8008 reading 2.499 V against the HP 34401A’s 2.49950 V DC

The AN8008 reading 5.001 V against the HP 34401A’s 5.00098 V DC, with a GW Instek GDM-8251A on the shelf above

The post-modification scope trace once more, 8.96 mV peak to peak

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 these modifications, it’s even better!