Prev: Intrinsic 1/f noise in ADCs and DACs
Next: Sad and Scary
From: rich on 8 Jul 2010 16:39
Hi All,
I am putting together a basic circuit to add 10X gain to the output
of an LM34 temperature sensor (to feed a voltmeter).

So have kludged together an op-amp in the classic non-inverting input
configuration. (1+R2/R1)

Single supply, 12V.

The problem is that the output shows high linearity error. The input
is DC so it is not related to frequency. At low input voltages the
output is lower than it should be and at higher input voltages it is
higher.

I have tried two op-amps, the LMC6081 and the LM324, they both show
show error.
I have tried different values of R2 and R1. I have put a series
resistor in the output. No change.

Rich
From: John Larkin on 8 Jul 2010 17:02
On Thu, 8 Jul 2010 13:39:48 -0700 (PDT), rich <rsoennichsen(a)gmail.com>
wrote:

>Hi All,
> I am putting together a basic circuit to add 10X gain to the output
>of an LM34 temperature sensor (to feed a voltmeter).
>
>So have kludged together an op-amp in the classic non-inverting input
>configuration. (1+R2/R1)
>
>Single supply, 12V.
>
>The problem is that the output shows high linearity error. The input
>is DC so it is not related to frequency. At low input voltages the
>output is lower than it should be and at higher input voltages it is
>higher.
>
>I have tried two op-amps, the LMC6081 and the LM324, they both show
>show error.
>I have tried different values of R2 and R1. I have put a series
>resistor in the output. No change.
>
>Rich

A single-supply opamp will not work very close to ground. Check the
common-mode specs.

Could the LM34 be oscillating? They love to do that. A little cable
capacitance can set them off.

John

From: Jim Thompson on 8 Jul 2010 17:27
On Thu, 8 Jul 2010 13:39:48 -0700 (PDT), rich <rsoennichsen(a)gmail.com>
wrote:

>Hi All,
> I am putting together a basic circuit to add 10X gain to the output
>of an LM34 temperature sensor (to feed a voltmeter).
>
>So have kludged together an op-amp in the classic non-inverting input
>configuration. (1+R2/R1)
>
>Single supply, 12V.
>
>The problem is that the output shows high linearity error. The input
>is DC so it is not related to frequency. At low input voltages the
>output is lower than it should be and at higher input voltages it is
>higher.
>
>I have tried two op-amps, the LMC6081 and the LM324, they both show
>show error.
>I have tried different values of R2 and R1. I have put a series
>resistor in the output. No change.
>
>Rich

Post your schematic here in LTspice format (text), or post a link to a
graphic. It's not at all clear how you have it lashed up.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
From: rich on 9 Jul 2010 15:30
Thanks to all for the help. Adding the output load resistor helped
but the problem went away when I disconnected the voltmeter. It ran
from a separate 5V regulator and was a large LED type drawing a lot of
current. I am wondering whether there was a ground loop or other
disturbance of ground affecting the linearity of the op-amp.


Rich
 | 
Pages: 1
Prev: Intrinsic 1/f noise in ADCs and DACs
Next: Sad and Scary