LMC7101: unity buffer amplifier - problem with output voltage.

Michal Michal1

Part Number: LMC7101

In one of our product we use LMC7101 amplifier as unity buffer amplifier configuration for VDC=2.5V reference voltage at the input.
Recently we've got a lot of amplifiers, which are unable to give 2.5V at the output (it gives 1.88V) for 3.3V power supply and 2k Ohm load.
The research shows that the maximum voltage that can be obtained at the output is Vdd-1.5V also for 5V supply voltage.
My question is this the correct behavior for this type of amplifier?
Could it be some faulty series?

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Micha,

strange, datasheet says that there shouldn't be any problem. Have you purchased the LMC7191 from an authorized TI's distributor?

Or is the LMC7101 oscillating eventually? Is a capacitive load connected to the output of LMC7101? Can you show a schematic?

Kai

0 Tim Claycomb over 4 years ago

TI__Genius 15410 points

Hi Michal,

The maximum output voltage on a 5V supply with a 2k ohm load is 4.7V. This is stated on page 7 of the datasheet under the Vo (output swing) specification.

However, the output swing limit is dependent on the output current required. This is displayed in Figure 7 and Figure 8 of the datasheet.

Can you post a schematic of your circuit that shows the load on the output of the op amp with labels showing the input voltage and desired output voltage?

Also, oscilloscope shots can be very helpful to troubleshoot an issue. Can you post some measurements showing the input voltage, output voltage, and supplies?

Thank you,

Tim Claycomb

0 Michal Michal1 over 4 years ago in reply to Tim Claycomb

Prodigy 100 points

Hi,

I have got simple circuit without any capacitor on the output.

The schematic circuit and oscilloscope shots below.

Ch1 – Power Supply, Ch2 – output (Vo), Ch3 – input (Vi)

Part number: LMC7101AIM5X/NOPB , Device Marking : A00A

Thank you,

Michal

0 Michal Michal1 over 4 years ago in reply to Michal Michal1

Prodigy 100 points

0 Tim Claycomb over 4 years ago in reply to Michal Michal1

TI__Genius 15410 points

Hi Michal,

It appears the images you attached in your post did not come through. To attach an image click the "Insert Code, Attach Files and more" link in the bottom right corner of the thread's screen. Please try to attach your images again.

Thank you,

Tim Claycomb

0 Michal Michal1 over 4 years ago in reply to Michal Michal1

Prodigy 100 points

sorry, but image paste from Word are not visible :/

0 Michal Michal1 over 4 years ago in reply to Michal Michal1

Prodigy 100 points

LMC7101_Measurement_TI.pdf

0 Tim Claycomb over 4 years ago in reply to Michal Michal1

TI__Genius 15410 points

Can you save the images as a .png or similar file type? Then click the "Insert Code, Attach Files and more" link in the bottom right corner of the thread's screen to insert an image. Then click the insert/edit media icon in the toolbar.

0 Michal Michal1 over 4 years ago in reply to Michal Michal1

Prodigy 100 points

I added attachment. Sorry for mess :)

0 Michal Michal1 over 4 years ago in reply to Tim Claycomb

Prodigy 100 points

0 kai klaas69 over 4 years ago in reply to Michal Michal1

Guru 140200 points

Hi Michal,

are you sure that it is a 2k load?

Kai

0 Tim Claycomb over 4 years ago in reply to kai klaas69

TI__Genius 15410 points

Hi Michal,

If you remove the load does your issue go away?

Is this happening to multiple units or just one?

Thanks,

Tim Claycomb

0 kai klaas69 over 4 years ago in reply to Tim Claycomb

Guru 140200 points

Hhm, looks kind of a damaged output stage...

Kai

0 Michal Michal1 over 4 years ago in reply to kai klaas69

Prodigy 100 points

Hi,

The measurements were done with 2k load.

Without any load the output range is not much better, about 2V for Vcc=3.3V.

It is happening for multiple units from one tape .

Thanks,
Michal

0 Tim Claycomb over 4 years ago in reply to Michal Michal1

TI__Genius 15410 points

Hi Michal,

Are you sure there is nothing else connected on your board? There are multiple Figures in the datasheet that show the output can get very close to the rails on a 3V supply (Figure 61 and 62) . It looks like you are using a voltage divider with R80 and R82 to apply you input signal. If you remove these resistors and apply a 1kHz or less 3Vpp input signal to the op amp what do you get? Please post scope shots of your measurements (input and output voltages). During the measurements make sure that the scope is set to the High Impedance setting and not 50ohms terminated. Having it 50ohm terminated causes the op amp to drive 50ohms which the LMC7101 cannot do due to the output current vs output voltage limits of the device.

Thank you,

Tim Claycomb

0 Michal Michal1 over 4 years ago in reply to Tim Claycomb

Prodigy 100 points

Hi Tim,

I'm sure there is nothing else connected, becouse I have build separate board with this amplifire.

I added measurement, as you suggested (ch1-output,ch2-input) and it is sure that somthing is wrong with output stage.

In my opinion it is fake series.

When we bought from another supplier it work fine and according datasheet.

Best Regards,

Michal

0 kai klaas69 over 4 years ago in reply to Michal Michal1

Guru 140200 points

Hi Michal,

have you purchased the faulty OPAmps from an authorized TI's distributor?

Kai

0 Michal Michal1 over 4 years ago in reply to kai klaas69

Prodigy 100 points

Hi Kai,

Supplier says that OP Amps are from China distributor. On the reel tape there are original TI's markings.

Michal

0 Tim Claycomb over 4 years ago in reply to Michal Michal1

TI__Genius 15410 points

Hi Michal,

I highly recommend only buying from an authorized TI distributor. You can find our authorized distributors here. For prototyping and smaller orders you can purchase devices directly from TI through the TI Store.

Thank you,

Tim Claycomb

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LMC7101: unity buffer amplifier - problem with output voltage.