Need a super fast SPDT switch that handles dual supply - Page 1
Forum section: electronics / beginners

author: AQUAMAN
author_rank: Regular Contributor
timestamp: May 09, 2014, 10:43:10 am
content: I need a switch that has switching times of less than 25nS.At the moment I am using one of these:http://www.analog.com/static/imported-files/data_sheets/ADG719.pdf.But the switch controls the input of an opamp (either to ground or the output of the preceding opamp) and the input of this opamp is periodically a negative voltageTherefore I seem to be getting quite bad leakage through the switch and the opamp goes off into saturationSo basically I need another switch like the one above but one that operates in +-5V.I found thishttp://www.analog.com/static/imported-files/data_sheets/ADG733_734.pdf.The ADG734 but I cant find another one like it in just a single package, or for +-5V siupply instead of +-2.5VDoes anyone have any suggestions?

author: tszaboo
author_rank: Super Contributor
timestamp: May 09, 2014, 11:27:42 am
content: Is there any reason not to use it, other that it has too many channels? Analog switches basically come from Analog devices (well at least the ones worth mentioning for sub megahertz precision analog signals), if you didnt find it at their website, usually it doesn't exist elsewhere. Others seems be be stuck behind with the technology, either they are slow, or the voltage is too low, or the Rds is too high, or they latch up badly.

author: AQUAMAN
author_rank: Regular Contributor
timestamp: May 09, 2014, 11:41:51 am
content: I dont particularly want +-2.5V as it means I have to include more components on the board to power it. It would also mean I have to change the gain on my preceding opamp to make sure its output doesnt go beyond +-2.5V, as it currently does.So I would prefer to run at +-5V.

author: AQUAMAN
author_rank: Regular Contributor
timestamp: May 09, 2014, 12:00:22 pm
content: Yes and also a massive 20 pin chip is a bit of a problem

author: tszaboo
author_rank: Super Contributor
timestamp: May 09, 2014, 12:44:45 pm
content: Quote from: AQUAMAN on May 09, 2014, 12:00:22 pmIf you use the filer, you will see that with 25ns, you only have 5.5V parts.http://www.analog.com/parametricsearch/en/10185?mtuid=MTS852zusJ2L-PC#10185?d=sel|0|-1|4683|546|2836|2838|164|-3|4259|4261&p4261=3.6e-9|2.5e-8&p4259=3e-9|2.5e-8Looks like it only exist in 16-TSSOP package which is like 6mm big, so not a monster package (unless you have a really small pcb)Alternatively, you can level shift the input signal (ie. instrumentation amplifier) and then you dont need negaive 2,5V

author: AQUAMAN
author_rank: Regular Contributor
timestamp: May 09, 2014, 02:52:07 pm
content: Basically its for an integrator.I want to integrator output to stay below +-5V for the ADC. If I shift the voltage it won't do that. Unless I shift the voltage again, but then I am making a circuit with a large amount of op amps.I'm also integrating over a period of about 300nS. So switching times of 50-100nS could cause problems when its close to a 3rd of my integration period

author: David Hess
author_rank: Super Contributor
timestamp: May 09, 2014, 04:19:13 pm
content: If you are switching the input of an integrator then the input signal is actually a current. Do it the old fashion way with current steering diodes or bipolar transistors. Then your switching time will be limited by the switching drive signal so 3.5 nanoseconds and faster with insignificant leakage, feedthrough, and charge injection become easy.It may possible to do this with FETs but you will probably need charge injection compensation like you would use with a sample and hold.

author: AQUAMAN
author_rank: Regular Contributor
timestamp: May 09, 2014, 04:34:36 pm
content: I dont know how to control the bipolar transistor from my dspAt the moment I use fibre optic with a pull up resistor, its easy. It would work if the switch didnt leak through the negative voltage applied to it sometimes when its off.