Question about V/I and SMU glitch-free voltage range-switching mechanism. - Page 1
Forum section: electronics / projects

author: desert
author_rank: Contributor
timestamp: March 24, 2023, 08:44:50 am
content: For semiconductor test, an important requirement for V/I and SMU isglitch-free voltage range-switching.A typical use case: force +4V@±5V range (to achieve with good resolution and accuracy), then change force voltage to +5.5V@±10V range. To realize the change, force DAC bit, voltage force gain and voltage measurement gain needs to be adjusted. And during range-switching, voltage drop should be avoided and voltage glitch should be minimized, because voltage drop and glitch could cause unwanted component behavior (for example, if voltage drop to 2.5V during range-switching, component could be reset due to too low voltage).I monitored output voltage during range-switching of a V/I instrument, the curve is perfect, it’s glitch-free and monotonous, no voltage drop. And it takes less than 50us to finish range-switching.From hardware point of view, there is always a time difference between DAC programming and voltage force gain (switch control) adjustment. If DAC bit adjustment is prior to voltage force gain adjustment, a voltage drop could happen. It’s very interesting to understand how glitch-free range-switching is implemented in instruments.Any detailed description or document about voltage range-switching implementation?

author: alm
author_rank: Super Contributor
timestamp: March 24, 2023, 08:55:39 am
content: Have you studied the theory of operations and schematics (when available) of older Keithley SMUs like the 2400 (theory of operations available) and 236 (full schematics available)?I'm not as familiar with what documentation is available for the HP 4142A/B and 4145A/B, but it may also be helpful.

author: desert
author_rank: Contributor
timestamp: March 24, 2023, 10:12:46 am
content: I have Keithley 236/237 SMU service manual. There are switches/relays for voltage and current range-switching, but no description on detailed range-switching implementation. And it’s not clear if Keithley SMUs support glitch-free range-switching.I verified that on a V/I instrument, not on SMU yet.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 24, 2023, 10:58:27 am
content: Chances are it depends a lot on the details for the range switching. Relays are naturally slow and may not allow for really glitch free swiching. Even the alternative photomos switches are not that fast (e.g. 0.1 - 1 ms range).For modern instruments they may just go the brute force way with enough resolution from the DAC, so that there is very little need for range switching while the DUT is connected and thus no need for glitch free switching. Other wise the regulator in the SMU may not be very fast and the DAC can be quite a bit faster, so that one could just use the DAC to compensate what even glitch the range switching causes.

author: desert
author_rank: Contributor
timestamp: March 24, 2023, 12:58:21 pm
content: Could you explain some details about using DAC to compensate range-switching glitch?And new-generation SMU and V/I instruments use digital regulation loop instead of traditional analog regulation loop. In that case, DAC value and amplifier gain (controlled by analog switches?) needs to be adjusted for range-switching. How to synchronize the changes so that there is no voltage drop or overshot during range-switching?