I am designing a microphone preamplifier circuit. I have used op amp OPA 37 which amplifies the signal coming from the microphone to be used by the ADC. The usable voltage range of ADC is 0-2.5V. I am using a 9V power supply and two biasing resistors at the input to obtain the output voltage swing. The bias point is at 4.5V. I want to know what can be done at the output of the op amp to get the bias point at 1.25V so that it can be used by ADC. In the actual circuit microphone replaces signal generator.
What about taking the output from C3 and putting that into a summing amplifier? http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html
In input is your audio and the other is 1.25V.
Since this is audio, you probably don't care about inversion of the signal.
You can put a voltage divider at the output of C3 (the ADC input). As long as the resistor values are high enough it should work fine, although it might put in more noise than you want.
As it is now, the ADC input does not seem to be biased (unless the ADC is self biased).
That op amp (OPA 37) is not suited to this application, and there is no reason to think that it will do what you want. If you look at the data sheet, its output voltage range (at +/- 15 volts supply) is only guaranteed to be +/- 12 volts with a fairly light load, and worse for small load resistance. That is, for a +/- 15 volt supply, the output is not guaranteed to get closer than 3 volts to negative supply.
If this number applies in your setup, you cannot produce an output of less than 3 volts.
Does the number apply? I don't know. The data sheet has no specs for a 9 volt (+/- 4.5 volt) supply, but I'd guess it does.
What you need is an op amp with "rail-to-rail output".
Additionally, you'll need to get rid of C3. This cap will produce audio signals which are centered at zero volts, and all the negative portions will be outside the ADC range.