SVR

The SVR board, offered since 2004, is a high quality platform that can support most monolithic voltage reference chips using the standard AD587 type pinout (including the AD586). Input power and output connections can be made with copper or gold plated clips such as minigrabbers to the copper test point or wires may be soldered to the corresponding pads. Schematic in pdf. The board is printed in 2 Oz. copper to minimize errors caused by trace voltage drops.

Step by step SVR assembly instructions (needs more detail on the trim components for later, but you can get the idea)

Power: The board applies input power to pin 2 and a common connection to pin 4. A 0.1 uF ceramic capacitor provides local bypass at pin 2. Monolithic reference voltage is typically sensitive to input voltage at about delta 100 uV (output) / delta 1 V (input).

The SVR requires an external (not supplied) single 15 VDC power supply, preferrably using a linear regulator. With no load on the SVR board, the board draws on the order of 2 mA. A 15 VDC linear power supply rated at 100 milliamps or more is recommended. For lower accuracy applications (over 100 ppm), two 9V batteries in series can power the board at about 18 V.

Inexpensive linear "open frame" supplies can be used. For example, 15 VDC models rated at 0.4A, 0.8A, 1A, and 1.5A are generally available at surplus and auction sites. Please remember to add an AC power fuse when using open frame supplies.

Trim: A jumper provides optional trim support.

jumper removed: (no external chip trim) This mode can be particularly useful as a backup calibration scheme where the trimpot may have inadvertently moved. With the trim jumper removed, pin 5 is left open and the output voltage is strictly the result of the laser trimmed calibration done during chip production.

jumper inserted: (trimpot is active) In this mode the board can be setup as a short term transfer standard. While 10 kilo ohms is most often recommended, the board uses a 200 or 500 ohm Bourns TM 20 or 25 turn trimpot (3296W) (R02) with fixed IRC TM 0.1% low tempco metal film resistors on both sides to improve resolution. Note that that tempco of these resistors is attenuated by the reference circuit (in other words they do not need to be 5 ppm / C or better).

Noise Suppression: 0.1 uF CK06 Kemet TM military spec low ESR ceramic capacitor (CK06BX104K).

Noise Reduction: A 0.1 uF Panasonic film capacitor (ECQ-V1H104JL) is connected to pin 8 and provides some improved high frequency noise performance. The R-C pole is set by an internal resistor coupled to the internal reference, typically on the order of 8 kilo ohms. Some reference families use a different pin for noise reduction (break out the Dremel tool for those chips).

Output: TP03 (Ref+) and TP04 (Ref- or common) double with the labeled output pads to provide output connections. Most monolithic reference chips can source 1 mA to 10 mA, but typically best results are achieved with loads of under 1 mA. Remember to account for IR (ohmic) loss when doing high precision measurements. Also, use care in terminations to avoid galvanic potentials that can cause small, but measureable errors.

Note the intentional ground break (a very thin clearance) between TP02 and the mounting pad. This provides an optional connection between board electronics common and chassis ground. To connect the two, make a solder jumper across the break.

Note: When we use a 200 ohm trimpot, the two Rs are select and test for each chip because the trim range is very small compared to the full range with the AD spec'd 10 k ohom trimmer. For DIY builders, we recommend a 1 k ohm trimmer and as a first approximation 7.32 k 1% to ground and 3.24k 1% to Vref. The results are slightly worse (a more sensitive trim and worse tempco), but still plenty good enough for most amateur applications.

We use a Mill-Max DIP Low Profile socket, Mouser no. 575-193308.

Mounting holes: 0.11" holes (.12" before plating) are provided for 4-40 mounting hardware. Contact us for available stainless steel standoffs, screws, nuts, and bolts.

Parts List:

AD587 chip, e.g. AD587KN DIGIKEY AD587KN-ND (the AD587LN was discontinued), or AD586 series chip for a 5 V board
C062K104K1X5CA MOUSER 80-CK06BX104K
ECQ-V1H104JL Digi-Key P4525-ND
3299W-1-201 MOUSER 652-3299W-1-201 (200 ohms, use 1 k ohms when a wide selection of select and test fixed trim R's are not available)
TRIM R IRC 0.1% MOUSER 66-RC55LF-D-3.24K (for some 587 production runs: only if using a 200 ohm trimmer, 2.87K or 3.01K, see manual)
TRIM R IRC 0.1% MOUSER 66-RC55LF-D-7.32K (see manual)
COPPER LOOPS #18 SOLID COPPER WIRE
2 PIN IDC HEADER
IDC SHORTING PLUG
Mill-Max SOCKT MOUSER 575-193308
solder

We try to be very clear about what this project is about. We are attempting to provide a low cost absolute voltage calibration point targeted to the amateur scientist, electronics hobbyists, consultants, and schools. Our SVR product is a high quality platform for the AD587 family (in our opinion) some of the best voltage reference chips ever made.

We call the SVR board a "transfer reference". The SVR board is a high quality platform for a monolithic reference chip, nothing more.

Our Fluke 732B, even with its FLUKE NIST traceable calibration, in the art of metrology, is generally referred to as a "transfer standard".

The SVR board is intended to give a small lab a way to verify their 10 V calibration point to better than 10 ppm (100 uV absolute). Our field tests have done considerably better, however 10 ppm is our conservative spec. Since the SVR board is rated at 5 ppm/c, the board should be used at about the same temperature as we calibrated the board (as reported) after a warm-up period and at about 15V. The reference chips we use are rated at 100 uV / V (power supply), however most perform much better.

The specifications are, for all practical purposes, the specs of the chip itself, and is not degraded below spec by our precision trim circuit which uses high quality IRC 0.1% 20 ppm/c trim resistors (the 20 ppm/c is per resistor, not tracking, and it is of the trim range, thus not deteriorating the board 5 ppm/c rating, and the 100 ppm/c trimmer is a small fraction of the IRC trim Rs). The boards are burned in for 300+ hours before final calibration. Final calibration is done with our Agilent 3458A (new in 2005) referenced to the 732B. (A "calibrated" hp or Agilent 3458A -alone- is no longer sufficient at 10 ppm absolute and below.)

The details of high precision use, 30 to 60 minute warm-up, use as close the temperature where we did the calibration, power supply voltage of 15V +/- 0.1V are all just to squeak the last ppms possible (to get as close as possible to our 732B voltage) using this relatively inexpensive method. At relatively low precision (e.g. 0.01%), these are all non-issues.

The AD587 series reference chips are rated at 15 ppm / 1,000 hours of operation. Very close inspection of most monolithic reference chip family specification sheets reveals a 1,000 hour rating generally ranging from 10 to 60 ppm / 1,000 hours.

All "L" grade and better reference boards and boxes are calibrated to within +/- 10 uV absolute and guaranteed to remain longer term (6 months) within +/- 500 uV (+/- .005%) of absolute as rated. Extra trim resistors are added to narrow the trim range for easier calibration. Calibration data is reported to 10 uV resolution (.0001%). The last digit of resolution will only be useful in the short term at the reported temperature and at the specified power supply voltage (15V). Under these conditions, it is possible to obtain short term transfer accuracy of better than +/- .0005% absolute (+/- 5 ppm).

Our Low Cost Voltage Transfer Project for Amateur Scientists, Electronics Hobbyists, and Schools, was Supported in Part by Analog Devices Incorporated. Please visit the Analog Devices website!

COPYRIGHT © 2005, 2011 JOSEPH M. GELLER

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