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interfacing fpga and opamp to make a fixed frequency variable amplitude sine

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abhishek fpga

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Hi All,

Im new to this site and putting my first query i hope i can get a quick solution to my problem .

I am a professional "FPGA circuit porter",we make a circuit in which 74164 is given 16 times the frequency to pin 8.The op generated and given to 8 resistors 120k,47k,30k,22k,22k,30k,47k,120k wth one of their legs shorted.This output is given to LM324 pin 3 with a resistor.Thus generating a sine wave of fixed frequency.

But when im replacing the op of 74164 with FPGA ,the offset of the op of LM324 is not 0V.

Please suggest any ideas.

Regards,

Abhishek
 
The difference is due to the output characteristics of a TTL device vs what the FPGA is (CMOS?) Do they both run on 5V or is the FPGA running on only 3.3V?
 
Is the FPGA CMOS?
 
Ok, the 74series TTL outputs pull to ~0.25V when low, and pull to ~4.2V when high. The CMOS outputs will pull within a few mV of 0V when low, and within a few mV of 5V when high. The LM324 was originally trimmed with the TLL levels; it needs to be retrimmed for the CMOS levels.

btw- this is an awful design.
 
btw- this is an awful design.
I agree.
The 74164 TTL logic IC is (was?) extremely old with spec's that are horrible for this circuit.
The LM324 is almost as old and has noise and crossover distortion that you do not want in a sinewave.
 
About 20 years ago I made a very low distortion audio signal generator with a circuit something like yours.
I used a variable frequency CD4018 Cmos 5-stage ring counter and 4 resistors to make a "digital sinewave" with 10 steps.
Then I used a variable-frequency switched-capacitor Butterworth lowpass filter IC to remove the high frequency harmonics.

I can't find the schematic anymore but the switched-capacitor Butterworth lowpass filter IC I used is not made anymore anyway (a Maxim IC can be used today instead).
 
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