Teledyne LeCroy ADP305 Differential Probe - Page 1
Forum section: electronics / testgear

author: RaymondMack
author_rank: Regular Contributor
timestamp: March 05, 2023, 10:06:31 am
content: I snagged a damaged LeCroy ADP305 for less than $20 USD and figured I could mod it into a "dumb" probe that I can hook up to my ancient HP scope (since the "smart" ProBus end of the probe was violently removed from the cable).For those that don't know, LeCroy's ProBus interface uses I2C and has an EEPROM that stores things like the attenuation and skew of the probe. It's a pretty cool setup that automates a bunch of boiler-plate stuff when connecting up a probe that, unfortunately, I can't use since I don't have a LeCroy scope. Que tears of blood. So anyways, I'm not really bothered that some baffoon ripped off the end of the probe. In fact, it's the only reason I managed to get it for such a good deal. For whatever absurd reason, HV differential probes cost as much as a new kidney... and most don't even have fancy pants EEPROMs doing all the heavy lifting when connecting them to a scope.Anyways, here's the probe as found.Here's what one looks like when it still has the ProBus connector on the end. Pretty.After removing the top label (which was surprisingly easy to do), there are four torx screws holding the top cover on. Pulling that off, we are greeted with this lovely view:I want to point out that there are an absurd number of adjustments for these probes (12 by my counting). LeCroy states, and I quote, "This probe has no adjustments." LOL. Clearly that's not the case.Moving on, we have three OPAs (TI THS3001CD), two linear voltage regulators (Ti LM317LM and LM337LM), two diodes, two relays and a whole bunch of passives. The input resistors are 4 megohm SGT26 by EBG. Never heard of them, but they look nice and apparently use ruthenium oxide on an alumina core.If you were wondering, the PCB has four layers.Those THS3001 cost (at the time of posting this) $6.661 in 1k quantities direct from TI. Yikes. Just those alone are worth more than I paid for the whole shebang.Also note the coloring of the wires and the silk screen text. Apparently LeCroy couldn't be bothered to match those up. And if you were paying close attention, you'll notice that not all the wires in the cable are used.By my counting, there are 9 wires: outer shield, coax (its jacket isolates coax shield from outer shield), red, black, white, grey, blue, green, and yellow.The PCB has 6 solder points: coax center conductor (J102, 1), coax shield + cable outer shield (J102, 2 and 3), red (J112), black (J110), grey (J111), and blue (J113).So, green, yellow and white are apparently not used here.And the business end. Ah, the symmetry.According to the manual, the ADP305 has switchable bandwidth (100 and 20 MHz), switchable attenuation (1000:1 and 100:1), and supposedly, there is an auto zero adjustment that can be used to remove the DC offset. I assume this last feature is done at the ProBus interface end of things, seeing how there is nothing on the board to do this. But this feature confuses me: If there are only two relays, and I assume U100 is for setting the BW and U101 for the gain of the two non-inverting OPAs, then how do they, and I quote, "disconnect the ADP30x inputs from the circuit under test, perform the auto zero function, and reconnect the inputs" with only two relays?Well, that's all for today. When I get some more time I'll figure out the wiring and how to acuate the relays. Once that's squared away I'll test the voltage regulators and see if the OPAs are still alive.

author: RaymondMack
author_rank: Regular Contributor
timestamp: March 06, 2023, 03:34:25 am
content: I finally figured out the wiring.The cable's outer shield is used for common and is tied to coax shield on layer 2.Red is +12V and blue is -12V, primarily on layer 3.The relays are tied to +12V. The BW and attenuation are then set by connecting the grey and black wires to common.The LM317L and LM337L are set to output ±9 V: 1.25 * (1 + 1500/243) = 8.97 V.Honestly, this probe couldn't be simpler to modify into a "dumb" probe. Just need two switches for the relays, some RG178 coax and a mini-DIN connector to get ±12V from my HP 1142A probe power supply or 54542C oscilloscope. Overall, I think the build quality is pretty nice. Much better than the Sapphire probe I used to have. But looking back at the Tektronix P5205A, I can't help but see obvious similarities in the housing and, in particular, the shield that goes around the PCB. I suspect LeCroy doesn't actually make these probes themselves except for possibly the ProBus end of the cable missing on my unit.https://www.eevblog.com/forum/testgear/tek-p5205-hv-differeantial-probe-teardown-btw-what-are-the-red-and-brown-wires/

author: alm
author_rank: Super Contributor
timestamp: March 06, 2023, 07:32:17 pm
content: Quote from: RaymondMack on March 06, 2023, 03:34:25 amThe Tek P5205A was definitely a Tek design: seeTekScopes postsby Steve "ditter2" who managed the design. And I highly doubt that Tek was making probes for Lecroy. But both Lecroy and HP bought active probes from Preamble labs, before Preamble was acquired by Lecroy (e.g. Lecroy AP022 <> HP 1152A and Lecroy AP033 <> HP 1154A). Lecroy has also rebranded the Sapphire SI-9000 as AP031. So it's certainly plausible that this probe was made by someone other than Lecroy, but it's unlikely that the Tek P5205 was made by the same manufacturer. It does appear that the Tek and Lecroy probesshare the same designer, though.The big difference between high voltage differential probes is usually CMRR, so comparing CMRR figures should give you a good idea if two probes are using the same design. For example theTek P5205CMRR is specified as > 60 dB at 100 kHz and > 40 dB at 3.2 MHz, while theADP305is specified for > 50 dB at 100 kHz and has no specification for CMRR at higher frequencies, but the "typical" CMRR graph from the manual shows about 32 dB at 3.2 MHz, so the Tek probe is clearly better than the Lecroy probe and not the same design.

author: RaymondMack
author_rank: Regular Contributor
timestamp: March 06, 2023, 11:53:44 pm
content: Hi Alm,I agree that the P5205A is the better design WRT to CMRR, but you also have to factor in HF loading, noise on the output and DC offset drift as well. That said, I think the Tek probe is better in all of these metrics as well. The MAX436 transconductance amp used in the Tek design likely helps in this regard, as opposed to using three THS3001 OPAs. Though I do like the use of relays in the LeCroy probe as mechanical switches can be damaged from rough handling.