LMV232

SNWS017C – DECEMBER 2004 – REVISED MARCH 2013

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Figure 23. Input referred Error vs. RF Input Power

Analyzing Figure 23 shows that three sections can be distinguished:

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At higher power levels the error increases.

•

A middle section where the error is constant and relatively small.

•

At lower power levels the error increases again.

These three sections are leading back to three error mechanisms. At higher power levels the detectors output

starts to saturate because the output voltage approaches the maximum signal swing that the detector can

handle. The maximum output voltage of the device thus limits the upper end of the detection range. Also the

maximum allowed ADC voltage of the baseband chip can limit the detection range at higher power levels. By

adjusting the feedback resistor RFB of Figure 21 the upper end of the range can be shifted. This is valid until the

detector cell inside the LMV232 is the limiting factor.

The middle section of the error curve shows a small error variation. This is the section where the detector is used

and is called the detection range of the detector. This range is limited on both sides by a maximum allowed error.

For low input power levels, the variation of output voltage is very small. Therefore the measurement resolution

ADC is important in order to measure those small variations. Offsets and temperature variation impact the

accuracy at low power levels as well.

DETECTION ERROR OVER TEMPERATURE

Like any power detector device, the output signal of the LMV232 mean square power detector shows some

residual variation over temperature that limits it's dynamic range. The variation determines the accuracy and

range of input power levels for which the detector produces an accurate output signal.

The error over temperature is mainly caused by the variation of the pedestal voltage. Besides this, a minimal

error contribution leads back to the conversion gain variation of the detector. This conversion gain error is visible

in the mid-power range, where the temperature error curves of Figure 23 run parallel to each other. Since the

conversion gain variation is acceptable, the focus will be on the pedestal voltage variation over temperature.

The pedestal voltage at 25°C is subtracted from the output voltage of each curve. Variations of the pedestal

voltage over temperature are thus included in the error.

The pedestal voltage variation itself consists of 2 error sources. One is the variation of the reference voltage

VREF. The other is an offset current IOS that is generated inside the detector. This is depicted in Figure 24.

Depending on the measurement strategy one or both error sources can be eliminated.

The error sources of the pedestal voltage can be shown in a formula for VOUT:

VOUT = VREF + (IOS + IDET) * RFB

Where IDET represents the intended detector output signal. In the absence of RF input power IDET equals zero.

The formula for the pedestal voltage can therefore be written as:

VPEDESTAL = VREF + IOS * RFB

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