LM7171 basic test

Hi Jen,

The issue you are describing with the LM7171 is very odd (the fact that your output is railed no matter what, on all units you have received). Production testing includes all kinds of tests on the device and if these have output stuck to one supply rail, they will never pass production testing and be allowed to ship.

With your "DC buffer application", can you describe the following please:

Feedback resistor value, load resistor and what voltage their other end is tied to, whether or not you have used decoupling caps next to the +/-15V supplies, how you are driving the non-inverting input (a voltage source?) and the series resistance with the non-inverting input (if any).

Do you happen to have any other standard pin out amplifiers that you can drop in the same buffer circuit to make sure we are only dealing with an issue with the LM7171 and not any setup / schematic causes?

Please let me know.

Regards,

Hooman

Hooman,

Thanks so much for the prompt reply and suggestions. Here are the test conditions:

1. Feedback resistance: tried 1M, 100k and 1k ohm.
2. Output: 100k to ground or open (to oscilloscope or multimeter)
3. +/- 15V are from regulators and 0.1uF by-pass capacitors are put between the supply pins and the ground.
4. non-inverting input: grounded. No series resistor.

Thanks,

Jen

Hi Jen,

Your arrangement seems correct and I agree with you that the output should not be railed.

More questions:

1. What is the +15V and -15V supply current draw / reading?

2. How many LM7171 samples have you tried and got the same results?

3. Do you have any other op amps with the standard Single pinout to try to rule out board issues?

4. What is the actual output voltage reading?

5. Can you probe the non-inverting input (pin 3) and the inverting input (pin 2) directly and share the readings? While you are at, might as well confirm +15V (pin 7) and -15V (pin 4) right at the pins.

Regards,

Hooman

Hooman,

Thanks for asking. Here are the test results:

1. The current drawn at the supplies are 5.5 - 6 mA.
2. There are 7 or 8 LM7171 in our setup and they all behave similarly, whether in the array or tested separately.
3. Yes with a LF411 it's perfectly fine.
4. Open-loop voltage output: 13.12V.
5. Non-inverting input: 0V, inverting input: 13.12V, V+: 14.75V, V-:-14.84V.

Thanks,

Jen

Hi Jen,

Regarding # 4:

You noted "open-loop voltage output" when in fact I was asking for the output voltage with the feedback resistor (e.g. 1k) in place (closed-loop). I'm thinking your answer is the same ~+13V closed-loop (please correct me if I'm wrong about this).

One detail is that LM7171 is stable for non-inverting gains of +2V/V or higher and you are operating at a closed loop gain of +1V/V. This is very unlikely to cause your output to rail against V+ (as you've been observing) and normally could be the reason for instability (are you observing any instability at the output? Are you using a fast oscilloscope to read the output). To test if low closed loop gain is an issue or not, you could try the schematic of Figure 66 (Av= +2V/V) to see.

Have you tried any lower supply voltages (e.g. +/-5V) to see if anything is affected?

Have you tried loading the output with 100ohm, or 1500hm as in Figure 66? I'm wondering if your scope probe capacitance is causing anything (loading the output could help things).

I've inquired internally within TI to find out what is the proper course of action for us to get these units from you and have a look at them and also to get you replacement units. I'll keep you posted.

Please keep me posted if you observe anything else.

Regards,

Hooman

Hello Jen,

I've been informed about the procedure for resolving your LM7171 issue, as stated below:

"We have no reports of “bad units” for this device. There a few ways this can be handled. You can send us a few units and we can test them. Or you can contact the distributor of where you purchased them for return instructions."

Please let me know which way you like to try.

Regards,

Hooman

Hi Jen,

I've been updated that if you've acquired your units through a distributor, standard procedure calls for you to go through the distributor to return the units. You will also have to fill out a "Customer Information Form" that they will provide to you.

Sorry about the inconvenience and we will make sure to figure out the issue with these units ASAP.

Regards,

Hooman

Hooman,

OK thank you very much for all the help. I will contact the distributors for return/exchange procedures.

Thanks again!

Jen