OpAmps which accept negative voltages have dual power supply, ie +5V and -5V. PSoCs have only a positive voltage.
There are several ways of getting around this problem.
One is to use a simple pwm output into a diode network that can develop a negative voltage.
Note solutions have some ripple, generally PSRR of OpAmp will take care of this. You
have to perform an error analysis to meet design goals.
Another way is to level shift input signals so they meet single supply CM range of PSOC inputs.
A cheap and dirty way is this method using 2 R's attached and terminated by Vdd. Note
Vdd has to be accurate, if you want accurate level shifts.
The Excel spread sheet to calc their values, attached.
121234.xls 12.0 K
One last comment, the actual physical reason has to do with production
CMOS processes, and inability to isolate, on a single substrate, multiple
polarities of voltage.
One other solution, is the obvious, there is a ton of low cost charge pump
circuits from all the major vendors. Maxim, TI, NSC, Rohm, Micrel.....
I have one question about the voltage used at psoc.
I am using a active low pass filter and i need to apply a function generator on input of my circuit. I think that i can not apply a negative voltage on my psoc but when i apply only a positive voltage, the signal on output is 0 because i am using a inverter operational amplifier. I tried apply only negative voltage on my circuit and the output is only positive, that is what i need. My question is: can i use this signal on input of my circuit ? with only negative voltage ? or this is harmful for micro ?
Best regards, Miguel Macedo.
sasa.png 114.9 K
Pin voltages must meet Vdda >= Vpin >= Vssa. Note that range applies
to analog outputs of PSOC as well. There is a small deviation from this
in using A/D, if its input buffer is bypassed you can exceed the rails by ~ 100
You have to translate /offset input voltages into PSOC. In case on Inv Amp
you have to offset it as well.
See the earlier posted methods of handling - inputs, eg. generating offset.