need a voltage limiter

[sudil99]

I have an amplifier is designed to amplify the signal so that output is max. 2.5 V because my ADC can read only upto 0 - 2.5 V. But my circuit uses +5 and -5 V supply.
So I need a voltage limiter which can cut out output beyond that range.

I used a resistor with zener diode of 2.7V and 3.3V, but zener diodes are not sharp at such ranges.
Voltage divider at output doesn't work because I need only positive output.
Currently I am using 4 diodes to limit voltage upto 2.8V and in parallel with 1 diode in opposite direction to limit voltage upto -0.7 V.

Is there any better solution?
(I cannot change my supply voltage because of other components)

Thanx

 

[crutschow]

What is the maximum input voltage of your ADC? If it's the supply voltage then you just need a series resistor and one diode to ground.

 

[sudil99]

my adc can read from 0 to 2.5V.
I dont know the absolute rating because it was not provided in the datasheet.

 

[ge]

If your amplifier is an opamp:
1) Can you adjust the gain of the amplifier so that your signal is < 2.5V?
2) or can you reduce the input voltage so that the amplifier output is less?
I'm very surprised that the adc does not give a "do not exceed" input level.

 

[sudil99]

Ya, I have set the gain of the opamp such that it amplifies the signal to 2.5 V. But since I am dealing with very small signal (EEG), sometimes noises are bigger than signal. In that case, the noises can go higher than 2.5 V.

 

[ge]

Can you reduce the DC bias voltage to the opamp to something like 4V?
Can you put a 3.3V buffer (or double inverter) between the opamp and ADC?

 

[crutschow]

my adc can read from 0 to 2.5V.
I dont know the absolute rating because it was not provided in the datasheet.

What is the part no. of the ADC. I would be very surprised if the maximum ratings info is not on the datasheet.

 

[sudil99]

I am using TSP65950 which has in-built ADCs

 

[ge]

I got the impression that you are using +/- 5 V supplies. The max "Main battery supply voltage" is 2.1 to 4.5 V. The min/max input is 0 to 1.0*Supply. See page 30 of the attached. OOPS, the "Manage Attachments" does not allow me to attach the datasheet.
https://www.alldatasheet.com/datasheet-pdf/pdf/254805/TI/TPS65950.html

 

[sudil99]

Sorry, that I did not described properly. I am using a board called gumstix, which has TSP65950 in it. So, it is internally powered by gumstix, which needs 5V supply.

I just have problem with my output connection to adc. I wanted to see if anybody could suggest me a simpler voltage limiter circuit. Right now I am using the diodes (in fig) which limilts voltage from -0.3 to +2.9 V