help on voltage amplifier

[warhorst]

hi, i am gonna need help on a voltage amplifier on the output of a pressure sensor.

The presure sensor i am using is a motorola MPX10D or a MPX5000 series, with a max of 6vdc input and a current input of 6mA. voltage offset is around 20 - 35mV.

The output(pressure sensor) is to drive the voltage amplifier and into a schimdt trigger(comparator) where it will input into a PIC(microcontroller) to run the chip.

i have read about a amplifier using an OpAmp. will that work?...if so what type of setup is best?

thanks

[audioguru]

Hi Warhorst,
Motorola says the maximum voltage for their MPX10D sensor is 6V that you have. Any more voltage and it produces errors due to self-heating. The typical voltage is only 3V.
Its range of output voltage is only 35mV, and its offset voltage is also near that. An ordinary opamp also has offset voltage but it changes with temperature changes and might create an error.
Since you need a gain of at least 1000, opamp noise level will also cause error.
To avoid electrical interference on its connecting cable, the circuit should have a well balanced differential input.

Therefore I recommend using an accurate high gain specialized IC that is used for low-level detection of heart electrical pulses called an instrumentation opamp. Most semiconductor manufacturers have them.

[warhorst]

so for the instrumentation amp schematic, is this setup ok? as shown in the picture.

and what do you recomend? And also the opamp part number for it.

Thanks

[audioguru]

Hi Warhorst,
For your differential amp circuit that uses ordinary opamps you need extremely well matched resistors that are impossible to find. Semiconductor firms make real Instrumentation Amplifier ICs with those well matched resistors inside! They also have accurate gain-determining resistors inside that are selected by strapping its pins.
Look in Google and it will link to Analog Devices and Linear Technology companies.

[warhorst]

ok thanks...i have found the burr-brown 118 and the AD620.

But how does the common mode voltage affect the sensor output?

[audioguru]

Hi Warhorst,
I'm glad you found good Instr Amps.
They have an excellent common mode rejection of 100dB (100,000), so you probably won't have to worry about it unless your gain is about 90dB (31,000) or more.
Your gain is going to be only about 1500.