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/etc... Amplifiers & Other Linear Forum
Input Independent Gain
I am using LM324 in non inverting mode to amplify voltage across a current sense resistor. The desired gain is 15 and I am using 22k as r2 and 1.5k r1 giving gain of about 15.5 by the formula 1+r2/r1. I am facing the problem that as input voltage increases the gain also changes which I want should be constant. Is there a way to get a constant gain independent of input voltage? Or should I try for another IC, if so please suggest.
It may be a stupid question but I really need the solution. Please suggest so that I can get a constant gain.
Thanks in advance
Perhaps you’d be interested in our current-shunt
monitors. Most of them have a fixed gain
and are designed to sense the voltage across a shunt resistor. Here is a link to the selection tool:
CSMs are typically used when the common-mode voltage exceeds
the power rails of the device, though they can be used within the rails as
well. Another device family you may be interested
in are our instrumentation amplifiers.
While there are many factors involved in selecting the
appropriate CSM, I would start with the following:
is the common-mode voltage range?
is the range of current you’ll be monitoring?
accuracy is required?
the current uni-directional or bi-directional?
e) Are you interested in high or low side sensing?
If you need help in selecting a part, please answer the
questions above and we'll do our best to assist you.
Precision Analog Applications
Sujit,May I assume the the other side of the current sense resistor is grounded? If not, a differential amplifier setup is needed.What is the maximum voltage on the sense resistor?What are the supply voltages on both pins 4 and 11?
With this information, I can solve the gain issue.
Regards,Ronald MichallickLinear Applications
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The gain is constant; there are other factors that are adding error terms-- making it appear that the gain is changing with input voltage.
To analyze this, much more information is needed; can you post a copy of your TINA circuit?
Regards, Neil P. Albaugh ex-Burr-Brown
I am very thankful to all for your suggestions. The main
The other side of the sense resistor is grounded and voltage
at pin 4 is 12V and pin 11 is 0V
I saw a note on current sensing and thinking of OPA 335 as
given here http://focus.ti.com/analog/docs/microsite.tsp?micrositeId=7§ionId=560&tabId=2181
What do you suggest on this?
Thanks for the additional information. Here is a link to a similar
View this post at http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/90756/314946.aspx#314946
Concerning the OPA335, there are a few things that I would like to point
out. First, while the input voltage
range does include ground, it can only come to within 1.5V of V+. The output voltage can come
to within 100mV of the rails (depending on load). The offset voltage, however, is 5uV maximum.
The post above highlights the use of the OPA320, whose input and output
voltage ranges are greater than the OPA335. The offset voltage, however, is 150uV maximum.
While there are other considerations to take into account (e.g. power, GBW, cost, etc.), the requirement for "maximum" accuracy will be best served by the amplifier with the lowest offset voltage. As a compromise you may be interested in the OPA333 (RRIO including ground on the input with 10uV max offset voltage). The BW of the OPA333, however, is just 350kHz.
You note that “…the voltage at pin 4 is 12V and pin 11 is 0V.” I assume you’re discussing the OPA335 as shown
in the link you referenced. This is
confusing to me as there are no OPA335 packages that have 11 pins. Also, pin 4 of the OPA335 is generally
V-. Nonetheless, please note that the OPA335, OPA320, and OPA333 all have a maximum recommended power supply voltage of 5.5V.
I am very thankful to you for your suggestion, The volatage at pin 4 and 11 is given for LM324 which I used initially, I am going to see the link which you gave
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