LMH6553: Clamp bandwidth, clamp slew rate

Hi,

Sorry for the delay in response.

To answer your questions:

1. The clamp slew rate means the rate of clamp voltage change at the output for a step input at the clamp threshold pin (Vclamp). As you know from the datasheet, the clamp threshold pin (Vclamp) is supposed to be  within Vcm < Vclamp < Vcm + 2V. The 100% overdrive refers to the Vclamp going from Vcm +0.1V to Vcm + 1.9V.

For example in the below schematic, the Vcm is set to 1.5V. As a result, 100% clamp overdrive would refer to the clamp threshold pin (Vclamp) going from 1.6V to 3.4V. Now, the rate of this clamp voltage change at the output would determine the slew rate of the clamp threshold pin which comes out to be 52 V/us.   

2. The clamp slew rate is independent of the actual signal path slew rate, which is specified at 2300V/us. The clamp function should work properly beyond 52V/us of signal path slew rate. Note that usually the Vclamp is set to a DC voltage instead of constantly changing step input, and the clamp slew rate is derived from this step input.

Best Regards,

Rohit

Hi,

The 100% overdrive in Figure 18 and 19 is for the main signal path performance where the output signal is clamped 100% to its actual output. For example, the middle output pulse in Figure 18 that swings +/-0.5V is 100% clamped with Vclamp = 0.5V, where the same output swing is supposed to be +/-1V (2 times+/-0.5V). Similarly, the outer output pulse in Figure 18 that swings +/-1V is 0% clamped (or unclamped) with Vclamp = 1V, because the output is able to achieve the full +/-1V without the clamp function kicking in.

This is different from the 100% overdrive in clamp slew rate specification, and I believe this spec applies for the clamp circuit itself.

Another way to look at this is to calculate the slew rate for the middle output pulse in Figure 18 with 100% overdrive, where the output swings from +/-0.5V. The slew rate calculation for this pulse (assuming 10% to 90% step) = 0.8V/1.6nsec or 500V/us. I think this slew rate value in Figure 18 is an order of magnitude (x10) higher than the clamp slew rate spec of 52V/us for 100% overdrive, right?

The other scenario could also be that you are right as to the clamp 100% overdrive does refer to the Figures 18 and 19. However, the engineer working on the datasheet at that time forgot to add a 0 at the end of the numbers for the clamp slew rate spec. Either way, I would trust the Figures 18 and 19, if that's the case, for the clamp slew rate spec to be much higher than 52 V/us.

Best Regards,
Rohit