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I have a circuit problem that I'm trying to find a solution for.

I have an 8 output IO expander which outputs 0V-5V. It is running analog switches which have a Vdd and Vss of +-2.3V.

Using the 5V right now is causing a dc offset and noise on the output of the analog switch.

I know I could just use an op amp to translate the voltage level, but that would require a lot of hardware for all 8 outputs.

I can't seem to find a level shifting IC that will accept a 5V signal and output a differential analog signal.

I've also thought about running it off of an 8 channel DAC. But that would be a costly solution.

Update for clarity: I am looking for a single IC that can accomplish the level shifting for all 8 signals instead of having to use a group of discrete components for each signal. OR, I am looking for an 8 ouput IO expander that has seperate supplies for communication with the microcontroller and IO outputs.

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  • \$\begingroup\$ Have you tried using a simple CMOS 4050 with supply rails of 2.3 and -2.3V ? the 4050 doesn't have protected inputs, so the input values can exceed the rails. You might need to bias your low signal down to the -2.3V rail, but maybe not. It depends on your actual driver output. \$\endgroup\$ – R Drast Apr 20 '15 at 18:11
  • \$\begingroup\$ For the future: you've given us some pretty good background info here (which is helpful, thanks), but you don't have a clear question. Keep in mind that you will get more direct answers if you ask more direct questions. \$\endgroup\$ – Greg d'Eon Apr 20 '15 at 18:38
  • \$\begingroup\$ Edited the original post thanks. R Drast, I picked up some CMOS 4050's yesterday and tried them out with +-2.3V rails and a 0-5V input. They are turn on at both 0 and 5V. I think I am going to try the PNP method. \$\endgroup\$ – mtb_engineer Apr 22 '15 at 22:08
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See below. When the input is at 0V, Q1 is on, the output voltage is pulled up to +2.3V. When the input is at 5V, the output is at -2.3V. Note the logic reversal.

You can use dual transistors etc.- they're not critical.

schematic

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  • \$\begingroup\$ Thanks for the answer. I'm sure this would work, but I'm trying to avoid using discrete components. I have 8 signals that need to be translated and this would lead to 24 extra discrete parts. \$\endgroup\$ – mtb_engineer Apr 20 '15 at 19:05
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    \$\begingroup\$ Using very common quad resistor networks and dual transistors, only 8 parts to do this function. Quad comparators would be the obvious alternative. Two quad comparators and 2 resistors to set up the threshold (and maybe 8 pullup resistors, depending on the comparator), so 3 to 5 individual components using the most common resistor networks. \$\endgroup\$ – Spehro Pefhany Apr 20 '15 at 19:57
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    \$\begingroup\$ You can also get quad transistor arrays. Go to Digikey and select Product Index, Discrete Semiconductors, BJT Arrays and then select type as 4 PNP (Quad). An MPQ3906 will run you about $1.60. 2 of those and either 2 16-pin DIP or 4 8-pin SIPs will do what you want. \$\endgroup\$ – WhatRoughBeast Apr 20 '15 at 20:24
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    \$\begingroup\$ Thanks for the solution. I tested it and found that it works great. My only concern is the number of parts required to implement it for 8 signals. I am going to try the line receiver first since it will only require 2 IC's. If that doesn't work, I will go with this solution. \$\endgroup\$ – mtb_engineer Apr 27 '15 at 16:49
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You might want to have a look at using a pair of quad RS-422 line receiver chips. The DS26c32 is one such device but there are others.

Each chip contains 4 identical line receivers. Connect the 26c32 Vss line to -2.3V, connect the 26c32 Vdd line to +2.3V, connect all of the input (-) terminals to 0V, connect each of the 4 (+) terminals to your input signals.

You can also get signal inversion for free if it is helpful - simply swap the receiver (+) & (-) inputs.

Most of these chips are fast - RS-422 is good for several Mb/s.

The downside is the current consumption. One reason that I'm suggesting the DS26c32 is that it consumes dramatically less current than the similar AM2632 or DS3486.

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  • \$\begingroup\$ Thanks for the advice. I will go pick up a DS26C32 and test it out. \$\endgroup\$ – mtb_engineer Apr 27 '15 at 16:48

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