High voltage to ADC  - Page 1
Forum section: electronics / projects

author: bbjk7
author_rank: Contributor
timestamp: November 09, 2015, 06:49:32 am
content: Hi all,So I would like to monitor the voltage at 4 to 5 spots and they range from 180 to 400VDC.. What would be the best way to accomplish this? I tried a resistor divider, but the component count quickly stacks up due to needing multiple resistors to handle the voltage & power dissipation (ideally i'd like to stick to 0805 components, but suppose i'll need to go up to 1206). Sure I could use really high values to divide it down, but the datasheets of the ADCs call for a maximum of 1.5k input impedance.. See below:To get a readable voltage, the combined resistance of the ladder can't really exceed 400k by much while keeping the last resistor as close to 1k5 as possible.This also dissipates a bit more power than i'd like.. nearly 1W for each node i'm sensing.I also thought of only using 1 divider ladder and switching the voltages on it one at a time... but that creates its own problems.. anyone know of a multiplexer that can handle 400V? heh

author: Psi
author_rank: Super Contributor
timestamp: November 09, 2015, 07:02:00 am
content: Use an opamp to buffer it. That way you can use megohm voltage divider resisors

author: Andreas
author_rank: Super Contributor
timestamp: November 09, 2015, 07:10:59 am
content: Hello,the 1,5K requirement usually results of either- leakage current of ADC (usually only a problem at higher temperatures)- sample time of the ADCso if you can live with either lower accuracy of your resultsor have only room temperature in your cirquita larger input impedance (around 10K should be no problem).A 10-100nF capacitor at the ADC input could also help for the sample time.(but not for the leakage current which can only avoided by usage of an OpAmp).With best regardsAndreas

author: Kalvin
author_rank: Super Contributor
timestamp: November 09, 2015, 07:14:57 am
content: If you place a well selected capacitor across the last resistor in the resistor chain, the capacitor will provide sufficient charge storage for the time the ADC samples the voltage. The capacitor will also provide averaging and noise reduction for free. The capacitor will reduce the maximum sampling bandwidth, though.Andreas was a little faster than me with his suggestion.

author: Neilm
author_rank: Super Contributor
timestamp: November 09, 2015, 07:01:36 pm
content: Be careful if you are putting a capacitor on the input to an ADC - I was bitten when there was enough current OUT of the input pin to cause the measurement to drift up.

author: Molenaar
author_rank: Regular Contributor
timestamp: November 09, 2015, 08:21:51 pm
content: Depending on the required sampling frequency, you might also switch out the ladder between measurements using a p-channel mosfet to reduce power consumption

author: tszaboo
author_rank: Super Contributor
timestamp: November 10, 2015, 04:35:09 pm
content: Use a ladder, 10 megaohm input, tap it at 1:100. That way you have 1.8-4V. Buffer it with an opamp and send it to the ADC. Capacitor will throw away your settling time.Or use an ADC with built in buffer.

author: Mark
author_rank: Frequent Contributor
timestamp: November 10, 2015, 07:14:19 pm
content: Use an instrumentation amplifier, they have really high input impedance so won't affect the loading on R15 and they have really low output impedance to feed into the ADC.