Industry/Application Area: Precision Instruments
PRND 1446 Bulk Metal® Foil Networks with MIL-PRF 83401 Characteristic "c"
VHP101 Ultra-High Precision Hermetically-Sealed Bulk Metal® Foil Resistor
VFD244 High Precision Voltage Divider Resistor
Analog-to-digital conversion at or beyond 24 effective, noise-free bits used to be an exclusive feature of higher end precision voltmeters. These rely on proprietary multislope techniques or even special hybrid components that are unavailable to engineers who seek to improve data acquisition performance in their own designs. With modern, integrated, high-resolution Delta-Sigma-ADCs that work with no more than a handful of external components, things got easier. But to guarantee the absolute best performance possible, no stone was left unturned and these 3 challenges were addressed:
- The on-chip voltage references in all available ADCs are not sufficiently stable
- A simple 2-point self-calibration feature is desirable
- The whole ADC can only be as good as its input attenuation circuitry
The on-chip voltage reference was substituted for an external, ovenized zener diode reference. Its most critical parameters like zener- and temperature-sensor-current are configured by hermetically sealed VHP101 resistors with their unparalleled shelf and load life stability. For the oven temperature the ratio stability of a resistor network is crucial, which is why a VFD244 voltage divider was chosen.
- To derive supremely stable and low-noise 5V and 10V reference voltages from a given zener diode, reference buffers with specific gains are needed. Here too the ratio stability of the resistors that are configuring operational amplifier gain is crucial. For that reason a hermetically sealed PRND 1446 network containing 8 individual V15X5 chips was used. The alumina ceramic DIP package couples the resistive elements thermally. Customer-specified resistance values are available, making this product immensely versatile.
- For the sake of simplicity the same PRND 1446 resistor network is used to set the gain of the fully differential input stage and to fine-tune its common mode rejection. Containing Bulk Metal® Foil Resistor chips of course this network inherits all advantages inherent to that technology, i.e. low noise, low voltage coefficient of resistance.
The User Explains
Configuring ovenized zener diodes with high end Vishay Foil Resistors has been a trusted best practice for over 30 years now with tens of thousands of devices today relying on roughly the same subcircuit that was also used here. At the time of writing no other manufacturer can deliver a lower temperature coefficient for the price of a VHP10* series resistor.
The same goes for the PRND 1446 resistor networks. The ability to specify resistance values freely, to have them thermally coupled and sealed hermetically for a lower price than that of 8 individual resistors of similar quality is very powerful and makes us a happy, returning customer.
Thanks to the CERN team for releasing the HPM7177 as an Open Source Hardware project and contributing to free education in the electronic engineering field.
Thanks to Vishay Foil Resistors for providing the aforementioned products as free samples.
A short video documentation about this project by Marco Reps