Easy DIY 5.5 Digit DVM + Volt Ref./Cal. (LTC2400+LTC6655 / SPI uC / Arduino)

[Kleinstein]

It is possible to add a temperature sensor and compensate for temperature effects. This can be more than just the reference alone, but also amplifier gain or a divider that can be temperature dependent. The problem is however that the references in a plastic case have more than just a simple temperature effect, but usually also some humidity effect and possibly an effect of the past temperature profile / hysteresis. A NTC can help, but it is not a magic solution for stability. It also needs some time and effort to measurent the correction function.

A 5.5. digit meter is usually no enough to really check a good bandgap reference - this is more something where one wants a good 6 digit meter with a reference that is considerably better than the DUT.

[Andreas]

Is it a practice to monitor voltage reference temperature and compensate for the temperature variation known ppm in the MCU or later on the PC?

It gives a improvement of ~factor 10.
But it needs some pre-requisites:
The reference must be repeatable (no hysteresis (<1ppm)) over the temperature range of interest (e.g. 10-40 deg C)
To get no additional noise from temperature quantisation the temperature steps need to have a good resolution (e.g. 0.1 deg C with a optimized NTC and 10 Bit ADC).
Further you have to select references with < 1ppm/K in the interesting temperature range to keep compensation steps below 0.1 ppm.

The best results I have with AD586LQ references (buried zener in hermetically CERDIP8 package).
Usually I get 2-3 devices from a batch of 10 which meet my requirements.

The other possibility would be to put a reference in a oven. (but this prohibits battery operation).

with best regards

Andreas

[jorgemef]

I purchased 5 AD586LQ on Aliexpress I was considering to use instead of ADR4050. I measure ~35uVPP noise. They seem popcorning if I am not wrong. Pin8 has 3.5V so they seem genuine but popcorning. Measurements where performed in breadboard with 1uF poliester cap in pin8. At least they have good TCR.
Alternatively I will use LM399H like circuit shown in attachement as still have a pair of 5K NOMC from when I built the voltage reference.

[Andreas]

I measure ~35uVPP noise.

noise without bandwidth is useless.
so how did you measure it (how many NPLC + measurement rate of your K2000?)
Did you put a 100 nF capacitor across the output? (could help if there are any EMI sources, but usually the dents go in negative direction).

I hope you did not use a switchmode supply for the references; otherwise I fear these parts are defective.

35uVpp is way too much for a DMM measurement.
By the way: the 1uF capacitor at the NR pin helps only to reduce wideband noise. (above 10 Hz).
Below 10 Hz it is nearly useless.

with best regards

Andreas

[Andreas]

Hello,

as comparison a quick + dirty measurement of a AD586LQ of my ADC28 with K2000 (10 NPLC ~600ms per measurement) battery supplied.
~7uVpp of AD586 + K2000 noise.

with best regards

Andreas