Matching signal to ADC input range - Page 1
Forum section: electronics / rf-microwave

author: danbr
author_rank: Contributor
timestamp: March 17, 2023, 12:56:25 am
content: I'm working with a analog sensor whose output is 2Vpk-pk, although our ADC is limited to 1Vpk-pk on its input. In terms of range limiting circuits, what would be best here given a required 500MHz bandwidth, and fast saturation recovery time <2ns?I've looked into zener clamps, although they seem more so for higher voltage applications. Additionally I've found some op amps which provide limiting/adc range matching, but most do not have large enough bandwidths. RF limiters might be an option, although one might limit the signal of interest as well.

author: Flux-Sucking Shunt
author_rank: Newbie
timestamp: March 17, 2023, 01:27:08 am
content: Will it always be 2V absolute peak? Wouldn't a resistive pad / voltage divider do the job?

author: danbr
author_rank: Contributor
timestamp: March 17, 2023, 01:33:30 am
content: I believe a pad/divider would reduce all signals, whereas I would like to limit the range to not over volt the ADC input and retain the original analog signal.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 17, 2023, 01:38:41 am
content: Quote from: Flux-Sucking Shunt on March 17, 2023, 01:27:08 amI would agree that a resistive pad would be a better idea than anything with non-linear components, although stray capacitance may be a problem with such wide bandwidth.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 17, 2023, 01:54:21 am
content: Quote from: danbr on March 17, 2023, 01:33:30 amYou would want to limit the input range so that the ADC does not effectively "clip" & produce a resultant digital signal which would not represent the full range of voltages at its input.If you "clip" the signal before the ADC input, it will have the same result as if you just overdrive the input.The output of a properly designed resistive pad should be simply a lower voltage copy of the input, but when applied to your ADC will be within its "dynamic range", & your resultant digital signal will be a faithful representation of the sensor output.Note: "All bets are off" if your sensor output has a standing DC offset.That would probably entail the use of active devices to properly fit the signal into your ADC input voltage range.

author: Marsupilami
author_rank: Frequent Contributor
timestamp: March 18, 2023, 06:16:20 pm
content: This maybe?https://www.analog.com/en/analog-dialogue/articles/rf-samp-adc-input-protection.html

author: radiolistener
author_rank: Super Contributor
timestamp: March 19, 2023, 08:19:20 am
content: if DC measurement is not required, it's better to use impedance transformer, it allows to add gain/attenuation with no adding noise. Just add 2:1 voltage ratio transformer on ADC input. But transformer frequency response may not be flat if you use it for wide bandwidth, so it may require to add digital FIR with compensation response which needs to be calculated for specific transformer at calibration time.if DC (or VLF) measurement is required, then you're needs to increase Vref voltage for your ADC in order to match with signal dynamic range. If ADC doesn't allow that, then you're needs to use different ADC.Another way is to add operational amplifier as a buffer before ADC, but it will add some noise and distortions and will reduce dynamic range which may be critical if you're use modern high resolution ADC. In addition, such a buffer (properly designed) may protect your ADC from voltage spike, but you're still needs to protect operational amplifier input according to it's specification.PS: don't confuse ADC input protection with gain/attenuation to match with signal dynamic range. Input protection is intended to protect ADC from unexpected voltage spike which is out of range for a normal signal dynamic range. It's goal is not to affect signal when it's voltage within expected range, and enable protection when something going wrong. Usually this is two anti-parallel Schottky diodes attached to ADC input for ESD protection. But such protection will not work properly if your signal voltage range overlaps ADC input voltage range. It just leads to open diodes and as result your signal will be distorted with clipping.

author: profdc9
author_rank: Frequent Contributor
timestamp: March 21, 2023, 03:09:58 am
content: If you just want to clip the signal, you might try to use a very low capacitance Schottky diode like a 1N5711 or the like. It has a forward voltage of about 0.4 V, and so with +/- 1 V supplies, you can keep the input voltage between +/- 1.4 V .