If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

• Resolved

# LMP7721: LMP7721 / TLV333: Input impedance and input differential / common mode resistance

Part Number: LMP7721

Hello everyone,

I'm working for my PhD prototype about high impedance sensors and I need to simulate and calculate some theorical values of them.

I'm looking for about different parameters about two operational amplifiers that I can't find:

1) Input resistance and input capacitance of the LMP7721

2) Differential resistance (between two inputs) of the LMP7721

3) Input resistance and input capacitance of the TLV333

4) Differential resistance (between two inputs) of the TLV333

These information is basic to study the small signal model circuit of the sensors that I am using, but It's not avaliable on datasheet and I don't know how I can obtain It or estimate.

Could someone help me?

Robert

• Hello,

Please, someone can give me a hand about how find or calculate these values?

Moreover, The LMP7721 output impedance or resistance isn't specidied in related documentation. Anyone knows how estimate it?

Best regards
Robert
• In reply to Robert Camos Vidal:

Hi Robert,

For the TLV333 the differential input capacitance is 2pF and the common-mode capacitance is 4pF. While we do not have data on this for the LMP7721, these are very typical values for input capacitance and the LMP7721 is likely very close to this.

Unfortunately we also do not have any data on the input resistance of these devices (this can actually be a very difficult parameter to measure, especially for low voltage devices). Given that these devices are both made on a CMOS process however, a good estimate for these parameters would be 100MOhms for the differential resistance and 6000GOhm for the common-mode.

We do not have any measured data on the output impedance of the LMP7721 and there isn't really a reliable way to estimate this. If you need this data this is a measurement we could perform, but it will take some time to acquire parts and setup the equipment.

Regards,

Zak Kaye
Precision Amplifiers Applications

• In reply to Zak Kaye:

Hi Zak,

I checked the input capacitance of TLV333 in the datasheet, sorry because I didn't see it in the first time. Also It's possible to fine the output resistance 2kOhms.

I've found the e2e thread that talk about the input common mode capacitance:
e2e.ti.com/.../300743

And the datasheet specified this in "Device functional modes" chapter, pag. 18/32
www.ti.com/.../lmp7721.pdf

Moreover, I did a test to estimate of these value and I obtained a really similar value 14pF, that It have some layout capacitive effect, as It is explained in the page 17/32 of the LMP7721 datasheet.

Taking in consideration your comments, If both amplifier are CMOS technology I'll supose that LMP7721 output impedance is similar of TLV333, around 2kOhms.

These information are enough For my PhD.