Nuts & Volts - February 2006

BY JOE GELLER

prototyping (gluing parts to a copper surface and air wiring them with all commons soldered to the copper ground plane) to a full printed circuit board. The layout is shown on the Nuts & Volts website ( www.nuts volts.com) for do-it-yourself PCB builders, or we offer a manufactured PCB (see parts list).

Low frequency analog construction is pretty forgiving. The only caution is to mount the resistors as close as practical to the op-amp input terminals (keeping connections relatively short). Also, be sure not to create any large thermal heatsinks near the LM399 leads, so as not to interfere with its temperature regulation system by needlessly draining off heat intended to maintain a constant temperature within the LM399 case.

The circuit will work with many standard types of dual op-amp that can operate reasonably close to the power supply rails. Some op-amps will give differing end scale values depending on how close their output stages can go to the positive rail or to ground. The recommended National Semiconductor LM6132BIN gives exceptionally good performance to both rails. The circuit also works fine — albeit with a very slightly reduced range — with the TI OPA2277 or the Analog Devices OP-297 dual op-amps. Older generation bipolar op-amps that cannot operate close to the rails are less desirable for this application. For example in this application, an LM1458 can only operate over a greatly reduced output scale starting around 6V. See the sidebar on op-amp rail-to-rail performance.

A socket is recommended both for troubleshooting and for experimenting with different types of op-amps. I recommend using all 1% metal film resistors for instrument type repeatable measurements, but any type of resistors with the correct values can be used just to see the circuit in operation.

I mounted the LM399 directly on the board without trimming the leads so that it stands up as high as practical. The LM399 can also be fashioned as a

probe using a twisted shielded cable. To make a probe, connect the oven (-) to one wire (black), the oven (+) to the other wire (red), and do not make a connection to the shield at the sensor side. At the circuit board side, connect the shield and the black wire to circuit common and the oven (+) wire to the junction of R1/R4 as if the LM399 were mounted on the circuit board.

Testing

■ The LM399.

CAUTION: THE EXPOSED

LM399 IS HOT. DO NOT TOUCH THE LM399. It is not hot enough to melt solder or even to boil water, but it will give you a nip if you touch it. DO NOT DO THIS PROJECT IN AN EXPLOSIVE ATMOSPHERE OR NEAR VOLATILE (flammable) CHEMICALS. To be safe, do not power it up anywhere you wouldn’t use a soldering iron (even though it runs quite a bit cooler).

Quick start instructions: Power up the MGTA board with a +15V (single supply operation) and connect a DMM (volts) to the MGTA board output. If you have an ammeter available to measure

the power supply load current to the board, the load current should be under 75 mA. The power supply load will stay near a relatively high level (about 60 mA or higher) for several seconds while the internal LM399 heater brings the oven up to operating temperature. Once at operating temperature, the board draws about 20 mA at no wind. Cover the LM399 with a Styrofoam coffee cup (I like Dunkin Donuts® medium cups). Use the trimpot to set the output to just above where your op-amp starts to operate (see the sidebar on rail-to-rail