matrix driver & MCP23017

[vannygen]

hi you, i would like to know if the MCP23017/MCP23S17 is able to give negative voltage oputput for logic 0 and positive voltage output for logic 1. I urgently need a driver or controller that lets me drive 16 photodiodes with negative voltage when turned off and positive voltage when turned on? I need +-1V voltage capability. anybody can help me?

 

[BeanieBots]

I don't know those devices but a quick scan of the datasheet suggests that it is a simple 0-5v logic device. So, in short no.
However, if you reference your devices to something other than 0v, then -ve would be possible. ie +/-2.5v. A potential divider on that could give +/-1v.
Perhaps if you described in more detail what you want to do there may be a simple solution.
Putting your device between two outputs would give +/-5v on each side.

 

[andrew_qld]

I have had a play with both the MCP23017 and MCP23S17 (code examples at http://andrew.meachen.net/ioexp.html ) and no, they don't do negative volts. How far negative do you have to go? Here a a few suggestions:

1) Run the controller IC from a seperate supply rail so a few volts below ground, so when it swings from 0 to +5 volts the photodiodes see (say) -4 to +1 volts. Ie the photo diode "ground" is at -4V

2) Maxim makes an IC that will simply change a positive rail into a negative one with almost no extra componants. You feed +5V into it and get (almost) -5 volts out of it. Feed +2V in and get -2V.

3) Use a MCP23017 to switch a transitor array or series of relays running from a seperate (negative) supply.

Really, its all about bias.