I am currently working on a senior design project that involves sampling an audio signal from an portable mp3 player using a TI TMS320F28023 DSP/MCU.
Using a unipolar 3.3V supply, I need to convert the input signal 0Vdc, ~200mVpp (ac coupled through 10uF cap) to a 1.6Vdc, 3.2Vpp signal with low impedance (several hundred ohms).
I attempted to offset the input signal with a voltage divider and then amplify the signal with an inverting op amp circuit, but this is not working. I'm assuming because of the unipolar supply.
I also tried an non-inverting level shifting amplifier, but again this gives me some strange results.
Sam
Google 'single supply opamp circuits'. For single supply opamp circuits, buffers, amps, filters etc.. since you're dealing with AC, they require a bipolar power supply, so, a 'virtual ground' is created at half the supply voltage. For a very simple voltage follower this would be exactly as you first tried....a signal, via a capacitor, connected to the positive input of an opamp. The positive input is biased at VCC/2 via a voltage divider, the parallel resistance of which determines the input impedance. So, using two 1Mohm resistors, gives an input impedance of 500k.
At 3.3v (very common now days for analogue) you'll need an opamp which can run at this voltage. Look for 'common input range' and output range, you may not need a
rail-to-rail input opamp, as you're only feeding it 200mv p-p ...after the capacitor, thats 100mv above and below the bias voltage, 1.6V meaning its 1.5-1.7V. This means the opamp you use must be able to input voltages within 1.5V of the power rails (most opamps can handle this).
However, as you need 3.2V p-p on the
output, the output of the opamp must be able to swing within 500mV of the power rails. So...its should be a rail-to-rail output opamp. Otherwise it clips horribly and distorts your signal.
Ther alternative to all the above? Use a higher supply voltage for the opamp. This will give the output headroom, allowing it to be the full 3.2V p-p as required. I could list some opamp part numbers, but frankly, just use what you have to hand. Check the specs for slew rate and bandwidth to make sure it can deal with the bandwidth of your signal (a simple LM358 isn't that good for audio). Then just power it with 5-12V. Still a unipolar power supply, but you can worry about getting hold of a R-R opamp later when you want to run the whole thing from the same 3.3v line.
*removed stuff about sigma delta ADC's, as your chip doesn't have one!*
I'm sure TI have provided documentation with example circuits for the use of their ADC's for this range of DSP's.
Blueteeth
Edit: just checked the datasheet for that chip, seems its not a delta sigma ADC, so you may need a higher pole filter for antialiasing...at least if you're after accurate sampling.