Since custom Vishay voltage dividers are becoming difficult to purchase these days in reasonable timeframes, I have recently purchased two off-the-shelf Vishay SMNZ resistor networks based on Z-foil technology for a precision voltage application that I am working on as a substitute for a Vishay Accutrim trimming potentiometer and a VHD200 voltage divider. Originally, I was going to borrow the voltage buffer concept from kj7e
https://www.eevblog.com/forum/metrology/ltz1000-10v-buffer/msg1435737/#msg1435737 that relies on a trimmer to achieve 10 DCV.
SMNZ:
http://www.vishaypg.com/docs/63117/smnz.pdfAccutrim:
http://www.vishaypg.com/docs/63056/12801285.pdfhttp://www.vishaypg.com/docs/63054/Accutrim_1260_RJ24.pdfVHD200:
http://www.vishaypg.com/docs/63036/vhd200_144.pdfMy interest in this resistor network technology began when TiN pointed out a significant disadvantage when using trimmers for precision voltage standards in a private email, which made me contemplate the use of a 4 resistor SOIC package instead. The disadvantage being that a trimmer can lose its position during transportation (rendering it pointless as a transfer standard), the trimmer TCR through the wiper is 25 ppm/°C (although promoted as '5 ppm/°C', which refers to the resistor itself), and thick film trimmers are not as stable as Vishay's Z-foil technology over time. I can ramble on about the trimmers, but you can read the datasheets for yourself. The advantage of the resistor network is that it is compact in size and will share a similar TCR. Furthermore, networks can provide a selectable fixed-resistance when the soldering connections or even with quality jumpers.
These are the various configurations that come to mind when applying a Vishay Y1747V0227AT9W (4 x 350 Ω SOIC-8 package):
configuration resistance value
R|R|R|R 87.5
R|R|R 116.67
(R+R)|R|R 140
R|R 175
(R+(R|R))|R 210
(R+R)|R 233.33
(R+R+R)|R 262.5
R 350
R|R|R+R 466.67
R|R+R 525
(R+R)|R+R 583.33
R+R 700
R|R+R+R 875
R+R+R 1050
R+R+R+R 1400
The average resistance step size of the 4 x 350 Ω resistor network is about 50 Ω until 583.3 Ω, which is fairly reasonable for my application. If the maximum resistance limit of interest is 262.5 Ω, then the average resistance step size reduces to 30 Ω. Even though the resistor network cannot compete with the continuous resistance values that a trimmer provides for a voltage divider, this package balances versatility and stability in a precision application.
For context, my resistor application is a critical component to a 10 DCV output stage for an LTZ1000ACH output board. The design was modified from TiN's FX-ref board:
https://xdevs.com/article/792x/You can view my resistance network simulation with Falstad (open the attached txt file with
https://www.falstad.com/circuit/) to get a sense of my application. The voltage divider network was designed to handle any LTZ1000 output and convert it into ~10 DCV.
1) The variable LTZ1000 input voltage range is based on the standard deviation of all available reported measurements (n = 16) I could find for the KX-ref, in addition to a multiplication factor of 4. This resulted in the range of approximately 7.047 to 7.216 DCV. Fun fact: my recent KX-ref is within this range (7.09295 DCV) but rather low in comparison to the mean. It is outside the limit of 1 standard deviation (7.11052 to 7.15267 DCV). The LTZ1000 datasheet provides a wide output range of 6.9 to 7.5 DCV.
2) Read the picture for a description of how to obtain ~10 DCV with the precision resistance network and a very small trimmer (if necessary). The simulation should be fairly intuitive. For example, with an input of 7.09259 DCV, the circuit can achieve an output of 9.951 DCV (0.5 % tolerance) without a trimmer. If a trimmer is necessary, then setting the trimmer to 70.2 Ω, it should be possible to achieve ~ 10 DCV. In this scenario, the optional trimmer would not contribute much to the overall TCR since the divider would be 10 kΩ / (703.33 Ω +
70.2 Ω).
Anyways. That is it for me. Hope I haven't confused anyone with my ramblings.
Enjoy!
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