LMV321: buffer design

user5866401

Intellectual 420 points

Part Number: LMV321
Other Parts Discussed in Thread: TLV9001

Hello;

There is an error about buffer design with rail to rail LMV321 opamp.

PWM frequency is 5kHz. Cut-off frequency 1.6 hz. Some results are below.

PWM Out 1 (V) out2 (V)

1 4,360 4,453

2 4,364 4,471

3 4,374 4,496

4 4,382 4,493

5 4,390 4,993

6 4,396 4,993

The buffer do not work on high voltage. WHY?

Best regards.

over 4 years ago

0 Michael Steffes over 4 years ago

Guru 47485 points

Well the LMV321 requires about 1V headroom to the positive supply on the input - so you are overdriving the input in the buffer configure,

You could attenuate the input by say 0.8X and then set up the amp for a gain of 1.25V/V to fix this.

0 user5866401 over 4 years ago in reply to Michael Steffes

Intellectual 420 points

Hello;

I can not see this info on datasheet.

What is your opamp suggestion using for above circuit as buffer?

Best Regards.

0 Michael Steffes over 4 years ago in reply to user5866401

Guru 47485 points

Well the old national datasheets are hard to read, it gives it as in input CM range in absolute voltage instead of headroom to the supply, but if you subtract the 4V input from the 5V supply you can derive a 1V headroom requirement. I tried to work through this very common confusion in this article, this site is messed up right now to where the hi res inserts don't open like they should, but the text is there. Hopefully this link (1 of 11 now) gets repaired soon.

https://www.planetanalog.com/input-and-output-voltage-range-issues-for-high-speed-amplifiers-insight-1/

0 user5866401 over 4 years ago in reply to Michael Steffes

Intellectual 420 points

Hello;

Thank you for your quick reply.

I tried something on circuit. And That is ok.

I insert a 100ohms resistor on feedback line. Please look at below picture.

And my problem is solved.

In adition do you have another sot23 packgage opamp suggestion for me?

0 Ron Michallick over 4 years ago in reply to user5866401

TI__Guru*** 139956 points

Hello User,

LMV321 is a RRO op amp ; rail to rail output

TLV9001UIDBVR is a RRIO op amp ; rail to rail input and output (same pinout)

The 100 ohm resistor is not a robust solution. I am surprised that it worked once.

0 user5866401 over 4 years ago in reply to Ron Michallick

Intellectual 420 points

Hello;

I found a book which is;

In that page;

So I connect Rf resistor.

Best Regards.

0 Ron Michallick over 4 years ago in reply to user5866401

TI__Guru*** 139956 points

Rf is for device's safety when inputs cannot tolerate and large VID (input voltage difference). LMV321 and TLV9001 can tolerate a large VID, so the resistor is not needed. The resistor will not improve the input common mode range on any op amp.

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LMV321: buffer design