LMH6518 output on oscilloscope

Hi Supratim,

To be clear:

You are trying to observe the main output of the LMH6518 on an oscilloscope to observe / analyze the waveform / response. Correct?

The differential probe on the two main output pins (14 and 15) should allow you to observe the differential output directly on the scope. I'm not sure why you are only getting a DC output while you say AUX output (configured as Figure 68) does produce a waveform!

Main and AUX output(s) are supposed to be identical copies of each other. If one has swing, the other should as well.

Questions:

1. What is connected to the Main output(s)? Could it be there is a short or other anomaly on those pins?

2. What if you observe each Main output individually (using a 10x probe)- do you then observe each output swing?

Regards,

Hooman

Hi Hooman,

1. One SMA connector is connected to each main o/p and nothing else. There is no short. It measures around 390 Ohm across out+ and out- when measured with a DMM. I could not find any anomaly.

2. Out+ shows a fixed 1.77V and Out- shows a fixed 0.16V when measured with a 10x probe. There is no swing. I expected a 1.18 V at each output as that is the voltage at Vcm pin.

Some observations:

I connected a 100 Ohm differentially across the main o/p and got a very low level signal. It was approx. 40 mV(p-p) while the Aux o/p was at 400 mV(p-p) (as expected from gain and attn. settings).

Regards,

Supratim

Hi Supratim,

I don't know what could be the issue you are experiencing with the Main output while the AUX output seems to be performing as expected!

Here are some notes anyway:

1. Do a power reset to the device to make sure to restore default power-on condition (Table 5) and to make sure there are no "unallowed" conditions (Table 6, and Table 7)

2. Probe the device input(s) to make sure the voltage is between 1.9V and 3.1V:

Also, capture each individual input with a scope (while triggering off a different point) to show the two inputs simultaneously.

3. Capture the AUX differential (or single) ended output on a scope.

4. Make sure power (5V) and ground at the device power (pins 3, and 4) / gnd (pin 12) pins

5. Change the device Preamp gain and Ladder attenuation (Table 4) and note if any of allowed settings produces / affects the output(s)?

Regards,

Hooman

Hi Hooman,

I followed your suggestions, but there was no change.

The input was 200 mV(pp) sine wave riding over a 2.5V common mode.

In the default power on condition, the Aux o/p was approximately a 100 mV(pp) riding over a 1.2V common mode. There was nothing at the main o/p except a fixed dc level (as mentioned in my previous mails) which is different from the applied common mode (1.2V).

When I terminated the main o/p differentially with 100 Ohm, a less than 20 mV(pp) sine riding over a 1.3V dc was observed at out+ and a similar sine wave riding over 0.72V dc was observed at the out-.

It is clear that even the applied Vcm is not coming to the main o/p. Is the IC damaged by any chance?

Am I doing anything wrong? Am I missing any termination? Please help.

Regards,

Supratim

Hi Hooman,

It is drawing approx 220 mA  from 5V. It's not beyond the data sheet limit. But I also feel that the part is damaged.

Thanks for your comments.

Regards,

Supratim