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RPL1116 (MSO1000Z) and PLA2216 (MSO5000) Active Logic Probe teardown

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Gandalf_Sr:
thmjpr
Thanks for the commentary. The AUC gates are cool but when I asked on TI's e2e forum about implementing voltage level translation using open drain outputs and pull up resistors, the expert advice was that, although this works, it's very power inefficient and the value of the pullup resistor needs to get smaller as the signals go faster.  They recommended the SN74AXC8T245 which is an 8-bit voltage level translator that uses uAmps and I've got one of these per 8-bit pod.

I will start a new thread soon and link to it from here.

[EDIT] First vn is up in the thread here

joeyjoejoe:
I think a lower cost pod is a great idea. As a hobbyist the 5000 series scope is at a nice price/feature point, but a LA at 200MHz just isn't needed. My cheap chinese one has been doing fine, so a cheaper pod would also be fine.

Gandalf_Sr:
My boards are due to arrive today but now I'm starting to worry that the LVDS output of the DS90LV047A will not work properly with the LVPECL input (assuming that is what it is) for the MSO5000 LA interface connector.  The LMH7322 outputs a differential voltage that swings between 0.35 and -0.35 volts  (see first picture) but I've made my DS90LV047A LVDS output circuit drive into a 100 Ohm resistor so my output is not going to centered around zero.  Have I fucked up?

I think I may need to drive into 2 x 50 Ohm with a center offset voltage but it's not clear to me from the DS90LV047A datasheet that this is needed (see second picture).

If this is a mistake I've made, I may be able to make these boards work by putting 2 x 50 Ohm resistors in place of the 100 Ohm one and then feeding a 1.2V supply into the center point of the resistors.

Any comments are welcome.

Vtech:
It's been a while since I've worked on the probe.

--- Quote from: Gandalf_Sr on March 13, 2020, 03:16:37 pm --- The LMH7322 outputs a differential voltage that swings between 0.35 and -0.35 volts  (see first picture) but I've made my DS90LV047A LVDS output circuit drive into a 100 Ohm resistor so my output is not going to centered around zero.  Have I fucked up?

--- End quote ---

Why do you think it's zero centered? It's RSPECL. At this VCCO voltage (2.4V) it should output levels H=1.3V, L=0.9V. These should be exactly LVDS levels (common mode voltage around 1.2V).

I can't measure it currently becouse I'm on a leave. I'll be back at work in two weeks.

Gandalf_Sr:

--- Quote from: Vtech on March 13, 2020, 05:37:28 pm ---It's been a while since I've worked on the probe.

--- Quote from: Gandalf_Sr on March 13, 2020, 03:16:37 pm --- The LMH7322 outputs a differential voltage that swings between 0.35 and -0.35 volts  (see first picture) but I've made my DS90LV047A LVDS output circuit drive into a 100 Ohm resistor so my output is not going to centered around zero.  Have I fucked up?

--- End quote ---

Why do you think it's zero centered? It's RSPECL. At this VCCO voltage (2.4V) it should output levels H=1.3V, L=0.9V. These should be exactly LVDS levels (common mode voltage around 1.2V).

I can't measure it currently becouse I'm on a leave. I'll be back at work in two weeks.

--- End quote ---
Thanks for the fast response. 

I was looking at the schematic in this post and the driver has +2.4 and -2.4 volts hooked up to it so I was assuming that the output was centered on zero.  Can anyone else confirm whether the voltage on the differential pair is centered on zero or 1.2V?

[EDIT] It does appear that, the way that the real LA probe circuit is connected, the LMH7322 Vcco is set at 2.5V which makes the H/L outputs run at +1.5V/+1.0V which is LVDS-compatible, so I think I'm OK (see picture).

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