EMG i.e electromyography amp, interface it with adc0808

[tux21]

i am working with an EMG i.e electromyography amp.
i use 2 x 9V batteries to get +9 and -9 supply with virtual ground in between

i am getting the final EMG output w.r.t the virtual ground i.e 9v above and 9v below it.

now the problem is to interface it with adc0808 and 8051 i have to have max peak to peak range of 5v w.r.t a gnd. Since adc0808 has only one gnd pin connection i have decided to put both Analog and Digital gnds together. I am taking the negative of the 18v supply battery as this gnd.To get the signal within 5v of this ground i use a voltage divider as in fig with R2 and R1 , R1 connected to the negative terminal.

Here v1 can be considered my emg output from amplifier which i wanna attenuate.
The cro can be thought as my adc where gnd probe of channel A is connected to -9v terminal.Some calculations will show that the cro input will range from 0 to approx 7v if taken w.r.t -9v.

My problem is i thought it would be better to add a buffer after the voltage divider so as not to load it down. But by inserting the buffer i get no ac output.I cannot say about dc shifts as i use pc soundcard oscilloscope which is ac coupled. Without using the buffer i get proper output from the adc0808. If i use OP07 instead of LM741 i am getting some positive portion of EMG . I am curious because i never thought i will have problems with a buffer.

ps. R1 is actually a pot from 0 to 500 ohms.

 

[FvM]

Re: opamp buffer problem

To amplify or buffer an AC voltage, any OP needs a bipolar supply, even with a modern rail-to-rail amplifier. Legacy 741 doesn't
work at all unless the input voltage raises several volts above the negative supply. So simply define the midpoint of your +/- 9V
supply as GND node and refer in- and output signals to it rather than to VEE.

 

[tux21]

opamp buffer problem

i think bipolar supply derived from unipolar supply makes no differnce to the op amp.
also i refer to Vee since i want all signal values to be only positive.

 

[tux21]

opamp buffer problem

any more suggestions???

 

[FvM]

Re: opamp buffer problem

i think bipolar supply derived from unipolar supply makes no difference to the op amp.

You think, but you obviously didn't check the 741 datasheet about valid input signal range. Believe it or not, it makes a difference.

also i refer to Vee since i want all signal values to be only positive

Your circuit shows an AC voltage source at the input, which is clearly bipolar.

 

[tux21]

opamp buffer problem

these opamps don't amplify or buffer ac with negative shifts like -5v in +-9 supply

 

[keith1200rs]

Re: opamp buffer problem

Is this what you want?

The values are important (even the 10k ones). You may be able to find "nicer" values providing the ratios are kept the same in each section - inverting input, non-inverting input. Other opamps would do the job - I just picked on at random.

Keith.

 

[tux21]

opamp buffer problem

thanks i will check with it