Input Independent Gain

Hello Sujit,

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:

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=426&uiTemplateId=NODE_STRY_PGE_T

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:

a)      What is the common-mode voltage range?

b)      What is the range of current you’ll be monitoring?

c)      What accuracy is required?

d)      Is 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.

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,
Ron Michallick

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,
Ron Michallick

Sujit;

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?

I am very thankful to all for your suggestions. The main requirement is

• 1.        To amplify a voltage from 0V to 0.3V.
• 2.       Maximum current from the sense resistor is 0.6A.
• 3.       Maximum accuracy is desired.
• 4.       Current is bidirectional.
• 5.       We are interested in low side sensing.

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&sectionId=560&tabId=2181

What do you suggest on this?

Thanks again

Sujit

Hello Sujit,

Thanks for the additional information.  Here is a link to a similar discussion:

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.

Hey Pete,

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

thanks again

Sujit :)