Andreas Gsponer

Hi Andrea, thanks for your reply!

Yes, we are measuring 2-terminal devices (particle detectors based on SiC p-n junctions with depletion voltages > 1kV), with capacitances ranging from 0.1 pF to 100 pF. Normally, we are just using a single test frequency around 100 kHz (and only the open correction). For the foreseeable future we are not going to perform 3-terminal measurements.
I understand that our application is a non-standard, but power electronics equipment seems to be the only niche offering measurement solutions up to 3kV. We obtained a CVU-3K-KIT for our C-V measurements because the cost of the triaxial 3kV connectors places it at a similar price point as building the bias tee ourselves.

The open correction procedure done by ACS (or as we implemented it in our own software, subtracting the impedance measured with lifted needles) is working so far with a K4200A-SCS. First I was under the impression that the connection compensation data was stored in the CVU internally, but if this is not the case, I understand now the implementation in Clarius / ACS.

At our institute we are sharing a K4200A between labs, so it is not always available. Being able to use a "regular" LCR meter (for example an E4980A) would be preferable. More specifically, I want to ask if it possible to use the CVU-3K-KIT bias tees with an LCR meter that has no internal bias tee (or a bias tee just for CVUHI, with CVULO fixed at 0V)?
I share you concerns with stacking bias tees, but I do not see any other way to obtain the +-10V control voltage if the LCR meter can not provide it on CVULO.

If this is not possible to implement, we will keep using the K4200A CVU for now.

 

This is a continuation of the thread in https://my.tek.com/en/tektalk/components/ea75857c-7640-ef11-b4ac-6045bd03a1eb.

The RBT-200 / RBT-3K require a +- 10V bias voltage applied to the AC input to close the AC switches, see CVU-3K-KIT user manual.
Normally, this control voltage is provided by the DC bias of the CVU itself. However, this has a major drawback: When a typical CVU performs an open correction, the DC bias is turned off and the open correction is performed with the AC switch open. Therefore, one needs to do a "manual open-correction", measuring the impedance for different frequencies and subtracting it offline (which is how Keithley ACS does it, see discussion in previous thread).
This can be quite cumbersome, and if the CVU can only source a voltage on the CVUHI side (and not CVULO), only one bias tee switch can be closed.

In this thread I want to ask if it is possible to use a second, external bias tee to provide the +- 10V to the RBT bias tees.
This would allow for controlling the bias tees independent of the state of the CVU and even to perform a "normal" open correction. Footnote 4 of the CVU-3K-KIT user manual states that for the "AC guard" mode, the +12V control voltage "Must be supplied by an external power source". Is there any more information on how one would use an external power source?

At low frequencies (100 kHz) it seems quite challenging to design a bias tee with a high enough isolation between AC and DC, especially if the DC resistance needs to be low enough such that the control signal of +-10V can be maintained within 250 mV.

Any information or recommendations would be greatly appreciated!


 

A small update:

I downloaded the trial version of ACS and tested the open compensation there:
By looking at the command log in the K4200 KXCI, I was able to see the difference in the open correction performed by ACS with and without bias tees.

Without bias tees, the open correction is performed directly via :CVU:CABLE:COMP:OPEN.
With bias tees, the open correction is performed manually, measuring Z from both CVUHI and CVULO for all frequencies.
This manual open correction is then stored in a file and applied in ACS after the measurement.

Does this mean it is impossible to use :CVU:CABLE:COMP:OPEN directly with a DC bias?
Storing the open correction in the hardware on the 4215 CVU would be much more convenient than having to acquire and apply the open correction manually.

 

Thanks for the quick reply!
We plan to integrate this measurement in our Python-based DAQ for our custom-built automated probe station, so I will be writing custom code to control the C-V measurement and not use ACS (which I currently do not have access to).
I never used the simultaneous voltage sourcing on CVUHI and CVULO, but this makes sense! With the following two commands the measurement now seems to work (-10V source at both CVUHI and CVULO):

:CVU:DCV:OFFSET -10

:CVU:DCV -10

However, if I perform an open correction the DC bias seems to be lost and set to 0V. Is there any way to prevent this?
I tried the :CVU:STANDBY 0 command to leave the DC bias enabled after the test, but this did not prevent the bias from being disabled.

If this is not possible, I could imagine performing the open correction manually (measuring Z for each range/frequency/terminal) and applying it offline, but this seems quite cumbersome.


Please let me know if this discussion is too off-topic for the Components & Accessories section and if I should re-post this question in Semiconductor Parameter Analyzers.

Here's a screenshot of the bias tee modes and the block diagram of the bias tee.

I am planning to perform high voltage (3kV) C-V measurements using the bias tees in the CVU-3K-KIT (see attached), a 2657A SMU and a 4215-CVU. The HV is applied to the DUT through a 2650-RBT-3K bias tee, while the low side is connected through a 2657A-PM-200 protection module and a 2600-RBT-200 bias tee.

I am struggling to understand the operation of the control voltage of the bias tees in the CVU-3K-KIT:
In order to perform capacitance measurements, the AC switch in the bias tee needs to be closed by applying a DC voltage of -10V on the AC input of the bias tee.
How is this DC voltage supposed to be generated? Using the integrated bias in the 4215-CVU or an external voltage source?
In the footnotes of the bias tee modes table (see picture), there is a footnote mention of an external voltage source.
However, an external voltage source would require an additional bias tee, right? So I assume the control voltage is supposed to be generated by the internal bias of the CVU?

I am familiar with the integrated 30V bias tee in the 4215-CVU, but in my thinking this bias voltage is always sourced either on the high (HPOT/HCUR) or low (LPOT/LCUR) side of the DUT, while the other side is at 0V.
In order to perform C-V measurements with these bias tees, I would need to apply -10V at both the high and low connectors.

In the K4200-KXCI reference manual, I found the following CVU command:

:CVU:DCV:OFFSET

which apparently can be used to apply a DC offset to the low terminal.
Does this mean I should use a 10V offset (:CVU:DCV:OFFSET -10) for the low terminal and set the high terminal also to -10V (:CVU:DCV -10)?
Are these DC voltages always applied, or just when doing measurements?
In order to perform an open correction, I would still need to be apply -10V to the bias tees in order to close the AC switches.

The documentation on the 2650-RBT-3K and 2600-RBT-200 bias tees is quite sparse, so I would be happy for some more information!