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| MS05000 Budget Logic Analyzer Probe Set Design |
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| ddffnn:
TI has a new level shifter SN74LXC8T245-Q1 that can run from 1.1 to 5.5 V. This might be a good replacement for SN74LVC8T245 on the 5V version. |
| Gandalf_Sr:
ddffnn Great find on the SN74LXC8T245, thanks! FYI, there is only one version of the probe bare PCB, it's made into a 3.3V or 5.0 volt variant by which voltage level translator you put in for U3 (with a few other slight component changes). I have done a cross-check and it does look like we can fit a SN74LXC8T245 for U3 on the Probe PCB with R10 changed to 0 \$\Omega\$ and then the PCB can run between 1.1V and 3.9V using the onboard Vadj LDO supply or you can feed an external 5V in (Abs max is 6.5V) to run the logic at 5V levels. With this change you lose the ability to go as low as 0.65V logic levels and, to get over 3.9V, you will have to supply an external VccA because you can only supply VccA with up to 3.9V Vadj from the Probe PCB. The SN74LXC8T245 datasheet also says it will support up to 420-Mbps for 3.3 V to 5.0 V I will buy/sample a couple of the SN74LXC8T245 ICs and try them out. |
| fozzyvis:
Looking at the schematics, trying to learn. Please correct me if I'm wrong. Am I correct to think that the purpose of this set is to: * Decouple/buffer (not sure of the correct term here, but those two should convey the idea-hopefully?) the inputs (the 200R resistor) * Slightly load the inputs so that their levels aren't floating (the 10k resistors) * Shift The inputs levels to a specific voltage (using the SN74AXC8T245) * Convert the single-ended inputs to a differential (LVDS) signal (DS90LV047A), which is what the scope inputs expect And the main difficulty is that this is a rather high-speed "product" so length matching and impedance of the power sources becomes critical? Right? I am just clueless about the note "Vref is connected to GND at remote end" (on the connector towards the scope). Why is the Vref connected to ground? Why on the remote end and not locally? |
| Gandalf_Sr:
--- Quote from: fozzyvis on December 12, 2020, 09:23:23 pm ---Looking at the schematics, trying to learn. Please correct me if I'm wrong. Am I correct to think that the purpose of this set is to: * Decouple/buffer (not sure of the correct term here, but those two should convey the idea-hopefully?) the inputs (the 200R resistor) * Slightly load the inputs so that their levels aren't floating (the 10k resistors) * Shift The inputs levels to a specific voltage (using the SN74AXC8T245) * Convert the single-ended inputs to a differential (LVDS) signal (DS90LV047A), which is what the scope inputs expect And the main difficulty is that this is a rather high-speed "product" so length matching and impedance of the power sources becomes critical? Right? I am just clueless about the note "Vref is connected to GND at remote end" (on the connector towards the scope). Why is the Vref connected to ground? Why on the remote end and not locally? --- End quote --- 1. There are 200 Ohm resistors in series as it's good practice to consider what happens if an over current or over voltage situation occurs 2. Yes, the 10k resistors are there to provide a load on the inputs. The level translator datasheet says that the inputs should not be left floating but I find it works just fine with no 10k load resistors 3. Yes, on the probe PCB, one of the key functions is to perform voltage level translation 4. Yes, the scope needs to see LVDS differential signals Other comments. - Trace length matching isn't super critical at 200 MHz but I have matched within a few mm for differential pairs. I have tested at 200 MHz clock rate and it works. - Vref is a ground that has a confusing label. I originally fed Vref from the scope, through the connector PCB, through the ribbon cable, and to the header on the probe PCB but then didn't use it so I made it an extra ground feed from the connector PCB; I'm just pointing out that the pin marked Vref is really a ground connection. The Vref that comes from the scope is the one that is supposed to set the logic level but it doesn't do anything, the voltage level translators take care of that based on the VccA voltage, it's detailed in the datasheets but you don't need to worry about it, just set the VccA for the system you're probing. |
| S. Petrukhin:
--- Quote from: Gandalf_Sr on December 13, 2020, 02:00:45 am ---The Vref that comes from the scope is the one that is supposed to set the logic level but it doesn't do anything, the voltage level translators take care of that based on the VccA voltage, it's detailed in the datasheets but you don't need to worry about it, just set the VccA for the system you're probing. --- End quote --- Hi, Gandalf_Sr! I checked both Vref outputs from the scope - it sets voltage to these outputs, which user selects in LA menu. In groups for 1-8 and 9-16 LA inputs. |
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