Is it possible to design an isolation voltage measurement circuit up to 200kHz? - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 27, 2011, 09:05:20 pm
content: Wish to have an isolation voltage measurement circuit with bandwidth up to 200kHz.Hope that the phase response is ~ 1 deg.Input voltage: +/- 5V. (able to measure AC and DC)Gain=1Do you think this circuit performance can be achieved?Cheers.

author: Zero999
author_rank: Super Contributor
timestamp: May 27, 2011, 09:34:56 pm
content: With an analogue opto-isolatior such as the HCNR200http://datasheet.octopart.com/HCNR201-000E-Avago-datasheet-30849.pdf

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 28, 2011, 06:56:56 am
content: Thanks. It's a good solution...

author: DaveW
author_rank: Frequent Contributor
timestamp: May 28, 2011, 10:56:35 am
content: Another option is something like this,http://www.analog.com/en/specialty-amplifiers/isolation-amplifiers/ad215/products/product.html(I know it doesn't have enough bandwidth, but it just happens to be open already for a project)This particular one is expensive as it's isolated up to 1500v, but there are cheaper ones with the bandwidth you need. Opto's are probably cheaper though

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 28, 2011, 01:48:27 pm
content: Quote from: DaveW on May 28, 2011, 10:56:35 amThanks. But this solution costs much more than my target budget.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 28, 2011, 01:57:39 pm
content: Quote from: onemilimeter on May 27, 2011, 09:05:20 pmI'm sorry as I realise that the specifications I placed earlier are too high for my application.In my application, the correct specifications should be as follows:Input voltage: +/- 5V (or scaled to be lower using voltage divider)Phase Shift @ 1kHz: less than 1 degreeGain: 1 (or 2) (flat from DC to at least 1kHz)Input to Output Propagation Delay (90%-90%): less than 5us

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2011, 10:19:15 am
content: Does it still have to work down to DC?

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 29, 2011, 12:54:38 pm
content: Quote from: Hero999 on May 29, 2011, 10:19:15 amOne of them will be used to measure the input dc-link voltage to a 3-ph inverter. It's a DC voltage and some "ripples" may ride on the DC voltage.Thanks.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 29, 2011, 01:05:07 pm
content: How about performing the A/D on the primary side and isolating the output?

author: Unknown
author_rank: Unknown
timestamp: May 29, 2011, 01:51:07 pm
content: That would seem the obvious solution to me, digital signals (especially something with unidirectional lines like SPI) are much easier to isolate.

author: Zero999
author_rank: Super Contributor
timestamp: May 29, 2011, 07:23:01 pm
content: Digital would definitely work but then the question is what level of accuracy is desired?Going from your response, I assume the signal is just positive i.e. monopolar? Bipolar would require a precision rectifier and transmitting the sign separately or biasing at DC which would need some zero trimming.How about transmitting the AC and low frequency/DC signals separately? 20kHz to 20kHz can be sent across an isolation transformer and all information below 20kHz could be sent digitally at a high resolution using an audio ADC.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 29, 2011, 11:21:41 pm
content: Quote from: NiHaoMike on May 29, 2011, 01:05:07 pmThanks. This solution is promising too...

author: onemilimeter
author_rank: Frequent Contributor
timestamp: May 29, 2011, 11:30:43 pm
content: Quote from: Hero999 on May 29, 2011, 07:23:01 pmI wish to build four voltage measurement channels.One is to measure the dc-link input voltage to a 3-phase inverter (the voltage is 0~50VDC, unipolar).Another three are to measure the phase voltages of a 3-phase induction machine (bipolar).I think 12-bit resolution is enough for my application.Do you mean "20kHz to 20kHz" or "20kHz to 200kHz" can be sent across an isolation transformer. By the way, what is the maximum frequency that an isolation transformer can take without distortion?Cheers.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2011, 08:56:32 am
content: Yes, it was a typo, I meant sending 20kHz to 200KHz through an isolation transformer and a lower frequencies via some other means.I think a 10-bit accuracy at 200kHz (a minimum sample rate of 400kHz) will be expensive.