Author Topic: Multi-channel oscilloscope by adding multiplexer  (Read 2717 times)

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Offline Mykeil

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Multi-channel oscilloscope by adding multiplexer
« on: April 21, 2017, 07:33:08 pm »
Hi,

I am looking for 1xN multiplexer to expand my oscilloscope channels.
The plan is to use Tektronix TDS7154 scope as ADC. It has 20GS/s sampling rate and 32Mpts record memory.
The collected data will be transferred to Labview by wifi and cut into channels.

It is hard to find multiplexer for this job. It must have:
- 1 MOhm input impedance,
- be enough fast: >1 us. switching speed,
- have usable bandwidth: >100 MHz,
- and minimum 30 channels.

Maybe someone did something like that and has some experience?
I spent lot of time searching and did not found.

RF muxes have good bandwidth but their switching speed is in milliseconds, since relays are used for switching.
General purpose muxes are fast. The switching speed is about 50us but the bandwidth is 1MHz.
I think FETs must be used for switching.


Thank you very much for your input.

Kind Regards,
Mykeil
« Last Edit: April 21, 2017, 07:38:15 pm by Mykeil »
 

Offline 0xfede

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Re: Multi-channel oscilloscope by adding multiplexer
« Reply #1 on: April 21, 2017, 07:59:42 pm »
Maybe a solution could be found using video crossbar switch IC such as AD8116.
Never tried it in such scenario but spec wise it should work (200MHz @ -3db, fast switching, 16 x 16 matrix).
The problem is that you need 30+ input front end and the switching should be somewhat synced with the ADC clock.

Hope it helps,
0xfede
Semel in anno licet insanire.
 

Offline Mykeil

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Re: Multi-channel oscilloscope by adding multiplexer
« Reply #2 on: April 22, 2017, 03:46:56 pm »
Yes, it is the only way, I think.

Thanks.
 

Offline David Hess

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Re: Multi-channel oscilloscope by adding multiplexer
« Reply #3 on: April 23, 2017, 07:49:06 pm »
Nothing you will find is going to work at a high oscilloscope input impedance of 1 megohm and that bandwidth; parasitic capacitance will just be too great for that many connections.  Video multiplexers operate at a low 50 or 75 ohm impedance.

So you either have to work with low impedance inputs or convert high impedance signals to low impedance with a buffer and then multiplex the signals for the oscilloscope input.  100 to 200 MHz buffers are not too demanding so this may be feasible.
 

Offline Mykeil

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Re: Multi-channel oscilloscope by adding multiplexer
« Reply #4 on: April 24, 2017, 07:17:41 pm »
The trace which connects all multiplexers outputs will be long. Adding impedance buffer on each channel will be necessary,  but multiplexer may be not needed since buffers can be switched on/off and can work as a multiplexer also.

Parasitic capacitance will be small if I will not add any ground plane. It will be only between traces thickness (C1) http://www.electro-tech-online.com/attachments/capacitive-switch-1-jpg.81868/

Besides that many multiplexers outputs connected together will give in sum nice amount of capacitance.

Thanks for the input.
 


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