FET input vs Resistive probe for high frequency signals - Page 1
Forum section: electronics / testgear

author: suspension
author_rank: Regular Contributor
timestamp: June 20, 2024, 05:45:30 pm
content: I am evaluating two used Lecroy scopes on ebay. One is DDA 3000 with 3GHz bandwidth with 50 Ω or 1 MΩ || 15 pF, 10 MΩ input that use BNC connectors. Other is DDA 5005A XXL, 5GHz bandwidth with only 50 Ω input which is not BNC.Both are from the same series. The comparison is here:https://cdn.teledynelecroy.com/files/pdf/lecroy_dda5005_datasheet.pdfMy main use case is to test a function generator which can output max of 500MHZ.For probes, I could either use a passive probe likehttps://www.digikey.com/en/products/detail/cal-test-electronics/CT4204/9645861or build a resistive probe as described in many places in this forum and outside (for example:https://www.sigcon.com/Pubs/straight/probes.htm).I have few clarifications however. The resistive probe is not only cheap, it seem to outperform FET input active probes at higher frequencies. What is the main downside?Secondly, would DDA 5005A XXL be a better candidate for resistive probes than DDA 3000 given its input configuaration?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 20, 2024, 08:08:28 pm
content: Active probes typically do better with high frequencies because they present less capacitive loading. It might matter in your application, or it might not be a big factor.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2024, 08:50:52 pm
content: Quote from: Conrad Hoffman on June 20, 2024, 08:08:28 pmLow impedance passive probes also present low input capacitance, but they also have a low input resistance at low frequencies.Active probes are preferred over low-z passive probes where the DC operating point would be disturbed by the low impedance of a low-z passive probe.As a practical matter however if you are only testing the 50 ohm output of a 500 MHz function generator, no probe is required. The 50 ohm output should be directly connected to the 50 ohm input of the oscilloscope with a 50 ohm cable. Otherwise the slower DDA 3000 which supports both 50 ohm and 1 megohm inputs will be more versatile.

author: suspension
author_rank: Regular Contributor
timestamp: June 21, 2024, 04:26:52 am
content: Thanks both. I think as far as my immediate and most important requirement is concerned, I can just connect FG output to scope 50 ohm input.Looking at the comparison of two scopes here:https://cdn.teledynelecroy.com/files/pdf/lecroy_dda5005_datasheet.pdf, I am trying to understand why the DDA 5005A XXL is speced at 5GHZ although the HW, specifically A2D conversion stage seem to be identical at 20GS/s in both models.

author: David Hess
author_rank: Super Contributor
timestamp: June 21, 2024, 12:49:42 pm
content: Quote from: suspension on June 21, 2024, 04:26:52 amMy guess is that the bandwidth is limited because the front end of the DDA 3000 has to support 1 megohm and 50 ohm inputs, likely because of a loss of fidelity. At best, there has to be an extra transmission line relay to switch between modes. Or maybe LeCroy did not trust BNC connectors above 3 GHz.I do not see anything about the bandwidth of the 1 megohm input. The choice of probe implies 500 MHz which would be typical.I am leery of DSOs which do not support peak detection, but the large record length may make up for it.I wonder what the acquisition rate is.