I'm currently designing an active filter which has to pass an analog signal between two cables (PLC). I'd however like to not let it pass in case the signal is already present (up to a certain amplitude) on the second cable, to avoid harmonic distortions and presence of multiple frequencies.

I was thinking of using a CMOS analog switch (as indicated here for instance Switching op-amp gain resistances - how?) at the output of the OAP which I'll turn on when I need it.

The biggest problem is that: the resistance is highly non-linear (because in the circuit I have I can have down to 2 Ohms of load) and I may have to drive up to 1A (the output amplifier is a power amplifier).

Another solution I though was simply using an inverting amplifier with two feedback resistances: one very small (no transfer) and one equal to the resistance put in front of the inverting input (unit gain).

The bandwidth is about 100kHz. No problems concerning gain, since I already amplify the signal before (using a static gain, or even a programmable gain operational amplifier). The problem is how to connect the output only when I need it. Another potential problem might be the small amplitude of the signal: 2V_RMS (a sine wave at about 100kHz).

Switching has to be relatively fast: less than 1us.

I was even thinking of using static relays but I'm not really sure either. Normal or even reed relays seem to be quite slow and deteriorate with time.

Basically I was looking for a switch with: low R_ON, high R_OFF (>> 1MegaOhm), quite fast, bidirectional (supporting ANALOG signals in sine form of low amplitudes), low THD, and possibly easy to control. If there can be 3,6 or more switches inside a SOIC (or other IC) package it would be very good, since I have very limited space.

I'm hoping to see if there are other (better) solutions available.

EDIT: I though it would not work, but what about using an OAP with a switch to control power supply ? I did a simulation with PSPICE and it seems to work. It only produces a small voltage offset output. This is not a problem since there is a decoupling capacitor in series at the exit, so DC voltage/current doesn't pass through. Could this actually work ?

OAP power off http://img140.imageshack.us/img140/7462/opampturnedoff.png

Relays should be able to take as much as 230V_AC when turned off (open circuit): there is a decoupling capacitor at output, but its impedance is pretty low at 50Hz/60Hz (200kOhm) compared to that of the open relay. I haven't found those with a low ESR yet.

  • \$\begingroup\$ Could you explain, what is an "OAP"? \$\endgroup\$ – The Photon May 5 '12 at 21:47
  • \$\begingroup\$ @ThePhoton - from the context I gather that he means opamp. I've never seen that acronym either. \$\endgroup\$ – stevenvh May 6 '12 at 6:25
  • \$\begingroup\$ My bad, I meant operational amplifier. Sorry for the trouble. \$\endgroup\$ – user51166 May 6 '12 at 8:00
  • \$\begingroup\$ When the signal is "already present", what is producing that signal? \$\endgroup\$ – The Photon May 6 '12 at 15:10
  • \$\begingroup\$ Also, can you ac-couple your output to the cable, or do you need output down to dc? \$\endgroup\$ – The Photon May 6 '12 at 15:11

The Analog Devices ADG812YRUZ has 4 SPST switches, 0.65 ohm typical on resistance, and comes in a 16-pin TSSOP package. It switches in < 30 ns, has a THD of 0.02%, and a -3 dB bandwidth of 90 MHz. However it can handle only 300 mA continuous. (I assume you could parallel them for higher current.)

  • \$\begingroup\$ Thank you. Not really a problem the 300mA continuous limit: the signal is transmitted over something like 20ms, then there will be a larger period of pause (I don't know exactly the length of this pause). The real problem is the peak value: it should be able to jump up to 1A-2A (on a 1-2 Ohm load), down to something like 5mA. It all depends on the load, which is variable due to external conditions. I really like the R_ON characteristic which is pretty flat. Any other analog switch with a little bit more of amps ? I don't really like putting 3-4 of those in parallel because it takes much space \$\endgroup\$ – user51166 May 5 '12 at 19:43
  • \$\begingroup\$ If you know other solutions as well, please suggest them. \$\endgroup\$ – user51166 May 5 '12 at 19:44
  • \$\begingroup\$ What about static relay ? Something like farnell.com/datasheets/358214.pdf support up to 2.5A. I just cannot figure if it's only for logic signals or if it works for analog ones as well. Any ideas ? Thank you. \$\endgroup\$ – user51166 May 5 '12 at 19:51
  • \$\begingroup\$ peak current for the ADG812 is only 500 ma. the problem with the relay (which would otherwise be a better choice) is that the switching time is a few ms, and you stated < 1 us. \$\endgroup\$ – tcrosley May 5 '12 at 21:02
  • \$\begingroup\$ hmm, another option might be discrete mosfets. have to think about that and get back to you. \$\endgroup\$ – tcrosley May 5 '12 at 21:05

You seem to be thinking about switching the output of a opamp, but consider instead having a power amp that always drives this line and switch its input signals. Now you don't need really low on resistance and can use more ordinary analog switches. The input of the power amp wouldn't need particularly high current.

  • \$\begingroup\$ You mean like a switch that switch from GND (if I don't want output) to the input signal (if I want to output) of the OP-amp ? That migh actually work, since the R_ON becomes very small compared to the input impedance of the amplifier. \$\endgroup\$ – user51166 May 6 '12 at 7:51

Out of what you proposed, an analog switch is probably the best bet. First let's look at your other two proposals:

simply using an inverting amplifier with two feedback resistances: one very small (no transfer) and one equal to the resistance put in front of the inverting input (unit gain).

This doesn't disconnect your filter output from the output cable. It just applies a (near) 0 V signal to the output. If there is another source trying to drive the cable, there will be contention. One source will be trying to drive 0 V, and the other source will be trying to drive a different value. One or the other of the two sources is likely to be damaged by this eventually.

what about using an OAP with a switch to control power supply ?

Again, this probably doesn't do what you want, but the reasons are more subtle. When you shut off power to your op-amp, it won't try to drive any voltage onto the output cable, which sounds like what you want. But, that doesn't tell the whole story. Most likely, your op-amp has diodes connected between its outputs and the power supply in the direction so that normally they'd be reverse biased. These diodes are there to prevent the op-amp being damaged under certain circumstances.

Now when the op-amp is powered off, these diodes connect the output to ground. So if your other source on the cable tries to drive a voltage above 0.5 V or so, the op-amp output diode will try to clip it to ground. In the case of a source capable of driving 1 A, again something is likely to be damaged in this case.

Now, what about using an analog switch, and what kind to use?

A simple MOSFET? The bulk diode will prevent a single MOSFET from blocking a bidirectional signal.

A solid-state relay? The SSR's I'm familiar with have switching time measured in ms, 1000x your requirement of 1 microsecond.

A reed relay? Reed relays able to switch 1 A are bulky and expensive.

An armature relay? Even bulkier and more expensive.

If you can't adjust your requirements, I'm guessing you will have to design something like an SSR (two back-to-back FETs) with your own driver circuit. Here's an example of an SSR schematic to show the arrangement of the FETs:

enter image description here

(image from a Vishay data sheet).

You'll eliminate the opto isolation (which you haven't said you need), and be able to reduce the switching time in proportion to your ability to provide gate drive current. Your gate driver will need to drive the gates several V above your maximum signal level.

Your problem will be how to fit this thing into your available space (which you've said is small, but not how small). Chips able to switch 1 A are typically not especially small, and even if they are they're likely to need additional board area for heat-sinking to avoid self-destruction.

  • \$\begingroup\$ I said space was only "small" because I fear you'd said it was impossible otherwise. Currently it should fit in 2cm x 2 cm x 2cm (can do 2 PCBs, but even with that it's a tight fit). I think I can go up to 4cm x 4cm x something but even like this I'm not sure it's enough. \$\endgroup\$ – user51166 May 6 '12 at 7:57
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    \$\begingroup\$ It's not that difficult to disconnect the output of an op-amp. It's difficult because you need bidirectional signals, high currents, fast switching time, and small space. If you can find a way to back off any one of those requirements, you can probably find a solution. \$\endgroup\$ – The Photon May 6 '12 at 15:06
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    \$\begingroup\$ Not sure what you mean by "add a load". Normally adding a load (at the far end of the cable?) would increase the current. Do you mean add an additional series device at the source? Because if that will work, why not just use a driver circuit with a limited output current? \$\endgroup\$ – The Photon May 6 '12 at 16:44
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    \$\begingroup\$ On ADG451, I'm not sure what your power supplies are, but note figure 5 in the datasheet: 4 Ohm Rds(on) depends on having +/- 15 V supplies. With +/- 5 V supplies, Rds(on) increases to near 7 Ohms, presumably also reducing current handling capability. \$\endgroup\$ – The Photon May 6 '12 at 16:57
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    \$\begingroup\$ On your later comment, I'd call that "adding a series current-limitting device to the source". The load is whatever you're trying to deliver power or signal to. \$\endgroup\$ – The Photon May 6 '12 at 17:02

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