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#1
04-02-2010, 02:41 PM
 arry Junior Member Join Date: Apr 2010 Posts: 26
voltage follower with an op amp

I am using an op amp MCP607 connected as a voltage follower (with negative feedback). Vdd is 5.0V and Vss is connected to ground.

When I pass 1V to (+)input I get 1V on the output, which is normal.
But If i pass 3.5V to (+)input I get 3.53V on the output, - well probably ok,
and if I pass 4.5V to (+)input I get 5V on the output. This is very strange to me.

This opamp has a 120dB Open loop gain and a 250uV input offset voltage.
Having read the article on op amps , I would expect to have a slightly lower voltage, say with a couple of mV, but not 0.5V.

What could be wrong?
#2
04-02-2010, 03:17 PM
 Jony130 Senior Member Join Date: Feb 2009 Location: Poland/Wroclaw Posts: 2,509

Well the max input voltage is equal Vdd-1.1V = 3.9V.
Read datasheet "Common mode input voltage"
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#3
04-02-2010, 04:37 PM
 arry Junior Member Join Date: Apr 2010 Posts: 26

so up to 3.9V the input voltage should be followed exactly, right ?

Then why 3.5V is followed as 3.53V ?
#4
04-02-2010, 04:48 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,043

Do you have any load on the output of the opamp besides the noninverting input connection?

Do you have a 0.1uF bypass cap from the Vdd terminal to GND?

With no load and a bypass cap wired as a voltage follower, your output voltage should be within 250uV of the input voltage - as long as you're within the specified common mode voltage range.

If not, your opamp may have been damaged or may be defective.
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#5
04-03-2010, 03:32 PM
 arry Junior Member Join Date: Apr 2010 Posts: 26

thanks , you turned out to be right my op amp was out of its common mode.

What op amp would you recommend for a good voltage follower to drive
#6
04-03-2010, 04:56 PM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,043

Something like an LT6003 would be good.

Basically, you're looking for rail-to-rail inputs and outputs, low offset, and low drift that is stable at unity gain.
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#7
04-03-2010, 06:23 PM
 kkazem Senior Member Join Date: Jul 2009 Location: Irvine, CA Posts: 160
Single Supply Unity Gain Follwer

Good luck
Kamran Kazem
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Kamran Kazem,
Senior Member of IEEE with 30+ years of power electronics, & general analog, digital, & microprosessor design. BSEE & MBA from UC, Irvine
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 The Following User Says Thank You to kkazem For This Useful Post: arry (04-14-2010)
#8
04-13-2010, 07:35 PM
 arry Junior Member Join Date: Apr 2010 Posts: 26

I will try to look for tlv342 but in the mean time I am playing with mcp6142 op amp.
Common mode from Vss-0.3V to Vdd+0.3V, input offset 10mV, rail-to-rail output.

So when my +input has 4.82V, on the output I get 4.92V. This is 100mV instead of 10mV. Is that normal?

The op amp is connected as a voltage follower,Vss to ground, Vdd=5V, no bypass cap, no load on the output pin.
#9
04-14-2010, 01:06 AM
 retched Senior Member Join Date: Dec 2009 Location: Baltimore, MD Posts: 5,198 Blog Entries: 14

with a gain of 10 it would be
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#10
04-14-2010, 01:34 AM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,043

From Microchip's MCP6142 page (link: http://www.microchip.com/wwwproducts...cName=en010453 )

MCP6142 - 600 nA, Non-Unity Gain Rail-to-Rail Input/Output Op Amps (MCP6141/2/3/4) (5/14/2009)
The MCP6142 is a dual 600nA op amp offering rail to rail input & output over the 1.4 to 5.5V operating range. This amplifier has a typical GBWP of 100kHz with a maximum quiescent current of 1 microampere

Rail to Rail input/output swing G>10 stable

So, without even looking at the datasheet, you should have known that you would need a gain > 10 for it to be stable.

You cannot operate that opamp at unity gain.

The 10mV offset really is quite high.

It does not meet two important specifications that I gave you.

I would not have selected that opamp.

The devil's in the details.

[eta]
I'll add a couple more items to the list:
1) Don't use a micropower opamp for your buffer/voltage follower.
2) use an opamp that has at least 1MHz bandwidth.
You want the opamp to bring the ADC input very rapidly to the voltage that is at the noninverting input of the opamp, and be able to hold that voltage steady while the ADC is sampling it, which means a low impedance output from the opamp.

A 10nF cap at the ADC input will help to keep the voltage stable while it is performing it's sampling, but you need an opamp that can respond rapidly enough, and supply enough current to keep that cap charged to the proper level.

If you have to drop one of my suggested specifications, then ditch the "low offset" specification, as you can compensate for the offset by subtraction in your ADC sampling routine.
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Last edited by SgtWookie; 04-14-2010 at 01:50 AM.
 The Following User Says Thank You to SgtWookie For This Useful Post: arry (04-14-2010)

 Tags amp, follower, voltage

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