Alternative high speed OP-AMP for Differential Scope Probes?  - Page 1
Forum section: electronics / beginners

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 09, 2024, 08:04:36 pm
content: Looking into building this 100MHz differential probe.https://hackaday.io/project/169390-a-10x-100mhz-differential-probe/detailshttps://github.com/paulvee/100MHz-Differential-ProbeThe problem is that the LTC6269 is $15+/each OP-AMP while the LTC6268 is around $10/each.Could I instead use 2x LTC6228https://www.analog.com/media/en/technical-documentation/data-sheets/LTC6228-6229.pdfinstead of both the LTC6269 & LTC6268? It's nearly half the price and appears to have better specs; lower voltage noise, higher slew rate, GBP, and more. They are the same footprint as well.

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 09, 2024, 08:14:23 pm
content: No because the input current bias is too high

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 09, 2024, 08:48:36 pm
content: Quote from: PartialDischarge on March 09, 2024, 08:14:23 pmCould you explain what you mean by this? The bias current of the op-amp is too high? I guess I couldn't simply modify the input resistors to fix this?

author: PartialDischarge
author_rank: Super Contributor
timestamp: March 09, 2024, 09:00:20 pm
content: It means that a lot of voltage will drop at the input resistors, for 1uA of bias and 1Mohm, you’d get 1V. Input opamp for these applications need to have pA input bias

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 09, 2024, 09:20:57 pm
content: Quote from: PartialDischarge on March 09, 2024, 09:00:20 pmI assume then that the opa859https://www.ti.com/lit/ds/symlink/opa859.pdfwill not work due to it NOT being a rail-to-rail OP-AMP? Or would there be other reasons for it not to work (aside from the obvious package differences and it having a "feedback" pin)?

author: KE5FX
author_rank: Super Contributor
timestamp: March 09, 2024, 09:38:14 pm
content: Does anyone expect this type of HF differential probe to have 1M input impedance, though? A few thousand ohms is generally fine for the applications these probes tend to be used for. Anything above that will be dominated by strays at high frequencies.nctnico's probe works wonderfully with an LMH5401-class opamp, whose Ib is north of 70 uA.

author: MathWizard
author_rank: Super Contributor
timestamp: March 10, 2024, 03:03:42 am
content: Just to build up my collection of op-amps, I bought some of these on ebay, no idea if they are real or not, or how usable they would be for some 1GHz probe's I was looking at. But that's ok, they'll be good for something fast.NEC uPC1678GV DC - 2GHzhttps://www.renesas.com/us/en/document/dst/upc1678gv-datasheet

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 10, 2024, 07:29:45 pm
content: Quote from: KE5FX on March 09, 2024, 09:38:14 pmI don't think he publishes a schematic/PCB of his circuit plus he doesn't have the item on his website anymore.

author: KE5FX
author_rank: Super Contributor
timestamp: March 10, 2024, 08:10:10 pm
content: Quote from: LooseJunkHater on March 10, 2024, 07:29:45 pm"Don't turn it on, take it apart!"Point being, it's important to distinguish the use cases and design goals for a single-ended active probe versus a differential one. A big selling point for SE active probes is that they relieve a lot of the dependence on the probe's ground connection, due to their super-high input impedance. They are normally built with discrete FET front ends but if you did use an opamp, you would indeed want very low input bias current.Meanwhile a differential probe is typically used on balanced signals that are themselves fairly low-Z, whether differential RF traces or LVDS, USB, etc. Their design is somewhat less critical in some ways (input impedance) but they introduce additional requirements that SE probes don't have at all, such as common-mode rejection.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 10, 2024, 10:38:52 pm
content: The typical differential probes start with a divider at the input to allow a high common mode signal and thus use with relatively high voltage. This divider needs to be relatively high resistance (like 1 M) to still offer protection and not run to hot when used with a higher voltage.If used directly at the high impedance dividers, the current noise would also cause quite some noise, as least at low frequencies.There should be 2 amplifiers that need to work with a high impedance later amplifier down the signal path could be OP with BJT based amplifiers. So U2 should be OK, U1.1 and U1.2 not.

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 11, 2024, 05:40:40 pm
content: Quote from: KE5FX on March 10, 2024, 08:10:10 pmSo I actually don't intend to measure RF/LVDS/USB signals but instead I want to measure the voltage signals across sense resistors. To do that, I assume either a single-ended or differential probe will work fine? I just need to avoid creating a short circuit (or blowing up my scope) when using multiple channels...

author: David Hess
author_rank: Super Contributor
timestamp: March 11, 2024, 07:56:00 pm
content: The low input bias current and high input impedance also minimize the effects of inductance in the probe allowing operation at higher frequencies. If you are making very short connections to a low impedance source than this is not necessary and a probe with low impedance inputs will work.High impedance inputs are just more useful for general purpose work. Low impedance probes are limited to low impedance sources and short lead lengths.