Some basic doubts regarding voltage follower

[asp87]

Hi,

I have some basic doubts regarding voltage follower configuration.

1. In the circuit diagram shown, the A+ input gets 4.87V. Since it's a voltage follower, output should also be 4.87V. Instead, it's 4.99V. Why is it so? I need to use this circuit to buffer a signal to the input channel of an ADC and hence want the voltage to be precise.

2. Is this the proper connection for a dual power supply opamp? In this circuit the opamp is not connected to ground anywhere.

Please clarify.

 

[LvW]

Re: Opamp Voltage follower doubt

Asp87, I think the first circuit is OK. Of course, the output voltage will NOT be exactly at 9.94/2 because of the resistor tolerances (47k).
However, did you forget to show the ground connection?

Question to the 2nd circuit: Really two different supply voltages?

 

[asp87]

Re: Opamp Voltage follower doubt

Hi,

Circuit 1:
The voltage divider will not be precisely 9.94/2 = 4.97V due to resistor tolerance. Agreed. But the question is about the output voltage of the opamp. For V follower, Vout is the voltage at its non-inverting terminal. As such, the output should be 4.87V, right? Then why do I get 4.99V (difference of 0.12 V)? Is this normal?

As for ground, the opamp itself does not have a ground pin. So where to connect it? This question arose in my mind and that's why I posted the second circuit mentioning it has not been connected to ground anywhere.

Circuit 2:
V+ and V- rails are different because I do not have access to proper power supplies right now: just two wall adapters providing 9.94V and 13.38V. If the expected voltage swing is from -7V to +7V, differing power supplies are not an issue, right? Anyway, in the actual circuit the V+ and V- will be +15V and -15V respectively.

 

[FvM]

Re: Opamp Voltage follower doubt

As such, the output should be 4.87V, right? Then why do I get 4.99V (difference of 0.12 V)? Is this normal?

Most likely you aren't measuring correctly. But without knowing involved power supply and instruments it makes no sense to guess about details.

As for ground, the opamp itself does not have a ground pin. So where to connect it?

The circuit is incorrect (or at least incomplete) in so far that you're annotating node voltages without specifying a ground reference. The node voltage values are only valid if you mark the negative supply node as "0 V" reference. In a real world, your DVM has two cables and the black "COM" probe has to be connected somewhere.

 

[asp87]

Re: Opamp Voltage follower doubt

Hi,

This is the updated circuit diagram.

Most likely you aren't measuring correctly. But without knowing involved power supply and instruments it makes no sense to guess about details.

Power supplies are two wall adapters rated 9V, 300mA and 12V, 2A. It gives 9.94V and 13.38V respectively. To measure voltage, I use a digital voltmeter.

The circuit is incorrect (or at least incomplete) in so far that you're annotating node voltages without specifying a ground reference. The node voltage values are only valid if you mark the negative supply node as "0 V" reference. In a real world, your DVM has two cables and the black "COM" probe has to be connected somewhere.

Have indicated the reference point in the updated diagram. Please go through it.

 

[FvM]

Re: Opamp Voltage follower doubt

A systematical error is introduced by the DVM loading the voltage divider. The usual 10 Mohm input resistance would however result in only 20 percent of the indicated error. So there seems to be a different cause.

Input capacitance of a DVM can cause problems (e.g. instability) when connecting it to an OP output. As a cross check, you could measure voltage difference between positive and negative input.

- - - Updated - - -

P.S. If high frequent noise is injected to the circuit by the wall adapter, there can be different kinds of non-linear effects. Hard to determine without an oscilloscope.

 

[asp87]

Re: Opamp Voltage follower doubt

Hi,

I get 0V instead of 0.12V between A- and A+ pins of the Opamp. Weird.
Can you please elaborate on the 'input capacitance of DVM'?

 

[godfreyl]

Re: Opamp Voltage follower doubt

I get 0V instead of 0.12V between A- and A+ pins of the Opamp. Weird.

Not weird. It means the resistance of your meter is 1M. When you measured the voltage on pin 3 of the opamp, the meter was in parallel with the bottom 47K resistor, causing the voltage to drop by 0.12V.

 

[asp87]

Re: Opamp Voltage follower doubt

Not weird. It means the resistance of your meter is 1M. When you measured the voltage on pin 3 of the opamp, the meter was in parallel with the bottom 47K resistor, causing the voltage to drop by 0.12V.

Oh yes.. How could I forget that! Thanks for reminding me.

If 'x' is the resistance of voltmeter, (47k || x) ohm in series with upper 47k resistance.

9.94 *[47x/(47+x)]} / {[47x/(47+x)] + 47} = 4.87
Solving, x = 1M.

 

[FvM]

Re: Opamp Voltage follower doubt

There won't be no voltage drop when measuring between Vin+ and Vin- directly.

 

[ftsolutions]

Re: Opamp Voltage follower doubt

Always be SURE to include at least a moderate value ( eg. 100nF) bypass capacitor right next to the op amp IC between the V+ and V- pins to reduce the effects of injected power supply noise