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LaPOD: Low cost Logic Analyzer probe for MSO5k, DHO900 and more!

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Mechatrommer:
can someone re-explain how 49.9ohm and 220ohm resistors form 1/10X attenuation? i read the original rigol la probe limit is 40V, 1k ohm input impedance? this is mind boggling.
i'm referring to this post: https://www.eevblog.com/forum/testgear/rpl1116-active-logic-probe-pod-for-1000z-series-teardown/msg2088667/#msg2088667

ebastler:

--- Quote from: Mechatrommer on December 02, 2023, 10:03:29 am ---can someone re-explain how 49.9ohm and 220ohm resistors form 1/10X attenuation? i read the original rigol la probe limit is 40V, 1k ohm input impedance? this is mind boggling.
i'm referring to this post: https://www.eevblog.com/forum/testgear/rpl1116-active-logic-probe-pod-for-1000z-series-teardown/msg2088667/#msg2088667

--- End quote ---

You need to look at the input side of the comparator, with a 10k/(90.8k+10k) divider. You were looking at the termination resistors for the differential output.

Mechatrommer:

--- Quote from: ebastler on December 02, 2023, 10:13:03 am ---
--- Quote from: Mechatrommer on December 02, 2023, 10:03:29 am ---can someone re-explain how 49.9ohm and 220ohm resistors form 1/10X attenuation? i read the original rigol la probe limit is 40V, 1k ohm input impedance? this is mind boggling.
i'm referring to this post: https://www.eevblog.com/forum/testgear/rpl1116-active-logic-probe-pod-for-1000z-series-teardown/msg2088667/#msg2088667

--- End quote ---
You need to look at the input side of the comparator, with a 10k/(90.8k+10k) divider. You were looking at the termination resistors for the differential output.
--- End quote ---
i thought the bottom side is the input, no? so that means, top side is the input? :palm:

ebastler:

--- Quote from: Mechatrommer on December 02, 2023, 11:12:16 am ---i thought the bottom side is the input, no? so that means, top side is the input? :palm:

--- End quote ---

In the schematic in the post you linked to, the inputs are on the left side of the comparator: The probed signal on IN+, the reference level (threshold) on IN-. The relevant voltage divider is on the IN+ signal.

The resistors you talked about (49 Ohm and 220 Ohm) are shown on the right side of the comparator. That's where the two differential output lines are. The resistors do not form a voltage divider; these are termination resistors.

Are you sure you should be considering your own probe design?  ::)

EDIT: Ah, you were now talking about the photo? Better use the schematic if you want to study the circuit design... But yes, on the photo the differential outputs are on the bottom -- which is why there are twice as many connections.

Mechatrommer:

--- Quote from: ebastler on December 02, 2023, 11:33:39 am ---Are you sure you should be considering your own probe design?  ::)

--- End quote ---
i'm currently trying to understand this thing inside out while i have time and interest. including evaluating pre-existing diy and original design how and why they do it.. just now trying to find comparator replacement to unobtanium LMH7322 (LMH7324 also not available from the sources i usually buy things from), not an easy task.. currently listing as possible/cheaper replacement ACDMP 607/562/564/567 but they are equally hard to find or expensive. LT1715 is cheaper, but probably crippled and 4ns prog delay may put some people off. what i dislike about pre existing is their form factor, large IDE connectors here and there and right angle connection, i prefer if possible much compact version with nice 3d printed casing... so i'm still evaluating, no conclusion yet, just learning.


--- Quote from: ebastler on December 02, 2023, 11:33:39 am ---In the schematic in the post you linked to, the inputs are on the left side of the comparator: The probed signal on IN+, the reference level (threshold) on IN-. The relevant voltage divider is on the IN+ signal.

--- End quote ---
ah sorry i didnt notice there is diagram attached below that post. thanks.

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