AD8436 - FAQ

Why is rms important?

Rms value has many applications, but its most important quality is its direct proportionality to the heating value of an ac waveform.

What does 'true rms' mean?

Low cost instruments often defined their ac measurement metric as 'rms'. However if you look 'under the hood', the conversion is 'average-value' from one or two diode rectifiers, calibrated to rms using a sine-wave. Unfortunately this approach is accurate solely for the calibrating waveform, not for other waveform formats.

What is rms?

rms is an acronym for 'root-mean-square'. Also known as the quadratic mean, rms is mathematically defined as a statistical measure of the magnitude of a varying quantity.

So far you've spoken only about power applications. What other types of applications would the AD8436 fulfill?

Any type of application using symmetrical 2 or 4-diode bridges. Examples of this would be low-frequency amplitude de-modulation, where the carrier frequency is relatively low (e.g., 50kHz). The AD8436 also features low errors with high crest-factor waveforms such as current spikes during the transition of switch-mode power supplies.

What is unique about the AD8436?

The AD8436 is unique for a variety of reasons. It’s the first product of its type to incorporate buffers at the input and the output, in a single package. The value proposition is exceptional for this reason alone, since prior art required three separate devices.

What does rms mean in the context of electrical engineering?

Rms is the square root of the arithmetic mean (average) of the squares of the original values. In this context, it becomes a very useful metric for continuous ac waveforms.

What then is an rms-dc converter?

An rms-dc converter is a device that provides an accurate dc voltage, equivalent to the rms value an ac waveform, without regard to its shape.

You advertise conversion down to 100μV. Why would anyone need this type of level?

As it happens, there are many applications for this kind of dynamic range. Safety protection includes detection of small leakage currents as with ground fault sensing of power lines. Certain types of rms-dc converters utilizing digital techniques are limited to a few milli-volts because the conversion ceases to operate at such low input levels.

What about power consumption for the added amplifiers?

In power sensitive applications the amplifier power-supplies are simply disconnected, preserving the core conversion.

My hand-held meter project has only a battery for a power supply. Must I provide a separate negative power supply for the AD8436?

A unique feature is the ability of the AD8436 to operate with either single or dual supplies. Simply select the appropriate pin connection.