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OP-AMPs

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hi,
I'm trying to use the AD8031 as a buffer for an ADC, but it doesn't seem to be working. The op-amp is a rail-to-rail op-amp, which i am operating from 5V and ground. The ADC works ok, but when i put a buffer (voltage follower) using the adc8031 between the source and the input it stops working. The ADC is the internal adc on the pic16f690. Does anyone know why this might be happening?
 

MikeMl

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Most Helpful Member
I'm guessing that you do not have the input pin to the PIC's ADC configured properly. If it is still a digital output in spite of your best efforts, the opamp cannot overwhelm the digital driver, while a power supply used for testing might...

Connect the voltage follower to the source, but leave it disconnected from the PIC. Does it work then?
 
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The source being used is just a potential divider. If i connect it straight to the ADC it works, but when i put the buffer in the middle it stops. The voltage from the buffer is correct until it is connected to the pic. When it is not connected to the PIC it is 2.5V, but when it is connected it is about 1V. If i connect the source straight to the PIC it stays at 2.5 V and the ADC works properly.
 

MikeMl

Well-Known Member
Most Helpful Member
Makes no sense unless the +Vs pin on the opamp is not connected to +5V.

What are the resistance(s) of the potential divider?
 
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I've tried potential dividers made from two 3 ohm resistors and two 300k ohm resistors. I've had no success with either. If i connect straight from the pot to the adc it works with the 3 ohm pot, but not the 300k because R is too large for the capacitor to charge. I'm sory i know this is strange problem. it has been holding be back for about a week now :eek:
 

Roff

Well-Known Member
Do you have good power supply decoupling on the 8031's power pin (0.1uF from the pin to ground, with very short leads). You also need a short connection from the negative supply pin to ground (preferably your ground is a plane).
It's possible that the AD8031 does not like the load presented by the A/D. You might want to try 10 ohms in series between the amplifier output and the A/D input.
 

MikeMl

Well-Known Member
Most Helpful Member
Ok, so if you connect a 2.5V source with a 1.5Ω source resistance to the ADC pin, it works. (Two 3Ω resistors in parallel)

If you connect a 2.5V source with a 150KΩ source resistance to the ADC pin, it doesn't.

If you connect the opamp with a source/sink current of ~ 25mA to the ADC pin, it doesn't work.

I still say the opamp and the 150K pot is fighting the DRiver inside the PIC. I still think the TRIS bit for that pin is in the wrong state. A 2.5V source with a 1.5Ω source impedance can source almost an amp, so it could overwhelm the PIC's output driver and still cause the ADC to see ~2.5V.
 
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Your right! that exactly what i was doing. For some reason i'd written h'10000000' instead of b'10000000'. i feel so stupid, but thank you so much. I really and very thankful for all of your help. You've save me a lot of stress and embarresment.

cheers,
ed.
 
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