Analog switch problem - Page 1
Forum section: electronics / projects

author: beandigital
author_rank: Contributor
timestamp: May 11, 2018, 10:37:25 am
content: I am using an analog switch (ADG719) to switch between two signal of 125kHz. Basically one signal is active then the other. I have a 125kHz square wave on the select input which means that the 1st input is sent out then the second and so on. Trouble is that the output isnt doing that. I just get an output (one of the other not sure) then about a 12ms gap then same again. If I slow down the select then I start to get a more constant output, although its not correct as its not selecting each in turn. I have looked at the data sheet but I cant really find the info that tells you how fast you can switch on the select. There are parameters that give info on the delays of the select input but they are a few ns. Does anyone know how I can find this info?Thanks

author: wraper
author_rank: Unknown
timestamp: May 11, 2018, 10:44:41 am
content: QuoteThere is probably something wrong with your control signals.

author: wraper
author_rank: Unknown
timestamp: May 11, 2018, 10:50:13 am
content: Also you need to be sure that signal does not go below GND and above Vdd

author: beandigital
author_rank: Contributor
timestamp: May 11, 2018, 12:25:50 pm
content: tOn and tOFF is just the delay from the input to the output. Also the select signal is within the voltage spec.

author: wraper
author_rank: Unknown
timestamp: May 11, 2018, 12:54:46 pm
content: Quote from: beandigital on May 11, 2018, 12:25:50 pmit's not. What on/off delay between in/out there can be if it's an analog switch.Quote

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2018, 01:40:14 pm
content: Quote from: beandigital on May 11, 2018, 10:37:25 amAre these frequencies synchronised? If they are, you've made a synchronised detector. If not, you will be down-sampling the signal frequency to whatever the difference is between the two frequencies. Either way, whatever comes out is not going to look like the 125kHz signal going in.What follows the switch?

author: beandigital
author_rank: Contributor
timestamp: May 11, 2018, 03:28:20 pm
content: The signals are from an image sensor. There is an odd and even channel. So you get the odd channel output then the even channel. So when one is outputting the other is inactive. I just toggle the select line to select one of the other. The inputs and outputs are buffered with opamps. I have found that if I adjust the timing of the select signal I can get the correct output. But I am not sure why this would be.

author: Andy Watson
author_rank: Super Contributor
timestamp: May 11, 2018, 09:21:05 pm
content: Ah, sorry, I assumed you were trying to sample a 125kHz signal with a 125kHz sample rate.However, you will need to synchronise the switching signal with your pixel clock. How do the two signal emerge from the sensor - are they simultaneous or are they interleaved ? I

author: beandigital
author_rank: Contributor
timestamp: May 12, 2018, 03:08:21 pm
content: There is a clock for each output. So the odd clock is enabled which then outputs the odd side. Then it is disabled and the even clock is enabled. So you only get one signal output at any one time. The clock is output for eight periods. The data is valid for the 7th period. But as I mentioned I only seem to get the correct output when I delay the select signal to the third period. If I set it to toggle on the 1st or 2nd I dont get the correct output. I dont really understand this as I thought the switch would just be letting through whatever is on its input. But it seems as though something else is happening that I dont understand.

author: Kalvin
author_rank: Super Contributor
timestamp: May 12, 2018, 04:58:04 pm
content: What is your VDD and what is your signal level you are trying to mux?

author: beandigital
author_rank: Contributor
timestamp: May 12, 2018, 05:08:08 pm
content: The analog switch is connected to 5V. The signal level can vary between 1.26V and 4.5V.

author: Zero999
author_rank: Super Contributor
timestamp: May 12, 2018, 06:35:56 pm
content: Posting a schematic will increase the chances of getting a helpful answer.