A Radio Frequency RMS Metering Circuit
- JC - Linear Systems
- 4 days ago
- 2 min read
RF Engineers know that temperature effects matter in RF measurement. This constant temperature substitution design tackles that challenge in a clever way using pico-amp diodes.
JFET Design Spotlight - A Radio Frequency RMS Metering Circuit
“When Linear Technology introduced the LT1088 RMS converter, it was packaged in a complicated iso-thermal DIP package to allow the published RMS measuring circuit application some working room. The problem with that application was temperature increasing with measured power allowing only limited precision and repeatability over environmental temperature fluctuations. However, by using the additional concentric 250Ω resistors, another control option was obviously available creating possible constant temperature operation regardless of input power measured. Here, the 250Ω resistors raise the Die temperature to about 40°-60°C where any additional thermal influence of incoming RF can be substitute balanced by complimentary reduction of the 250Ω power input. Common mode voltage of the LT1088 D1&D4 diodes sets the temperature and differentially balances out the input RF at 50Ω with a DC equivalent power at 50Ω. The Voltage across D1&D4 are therefore constant for the input measurement range of 80mW. I talked Jim Williams out of some plastic packaged LT1088’s to prove that the expense of isothermal packaging was unnecessary. Accuracy improved over 10-fold from the original application circuit. As is the case for many of these instrument circuits, Clamp and level fixing diodes used with FET input Op-Amps by habit use pico-amp diodes like the LIS PAD-1‘s. By the way, Dual PAD pico-amp diodes have a little known feature of exhibiting significantly less nodal leakage than the Datasheet illustrates when used in symmetrical clamp bias function as shown in the earlier published 10Kv amplifier protection. These on DIE diode pairs function similarly to the gate bias cancellation of dual pico-amp JFET’s illustrated in the Accelerometer circuit presented earlier. Close proximity allows uniform doping distribution and nearly perfect matching for Dual PAD diode parts so symmetric bias currents balance to near nothing. For me, it’s a time-tested habit.” Kirkwood Rough
Why this matters:
Improved measurement accuracy through thermal stability
Symmetrical diode behavior enables ultra-low leakage performance
A great example of where Linear Systems PAD pico-amp diodes shine
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