LMV232TLX datasheet(10/13 Pages) NSC

10 / 13 page

Application Notes (Continued)

The detection curves of Figure 2 show the detector response

to RF input power. To show the complete dynamic range on

a logarithmic scale, the pedestal voltage (V

PEDESTAL) is sub-

tracted from the output. The pedestal voltage is defined as

the output voltage in the absence of an RF input signal (at

25˚C). The best-fit ideal mean square response is repre-

sented by the fitted curve in Figure 2. The input referred error

of the detection curves with respect to this best-fit mean

square response is determined as follows:

•

Determine the best-fit mean square response.

•

Determine the output referred error between the actual

detector response and the ideal mean square response.

•

Translate the output referred error to an input referred

error.

The best-fit linear curve is obtained from the detector re-

sponse by means of linear regression. The output referred

error is calculated with the formula:

Error

dBV = 20*log[ (VOUT-VPEDESTAL)/(KDET*PIN)]

Where,

Conversion gain of the ideal fitted curve K

DET is in V/mW

and the RF input power P

IN in mW.

To translate this output referred error (in dB) to an input

referred error, it has to be divided by a factor of 2. This is due

to the mean square characteristic of the device. The re-

sponse of a mean square detector changes by 2 dB for every

dB change of the input power. Figure 3 depicts the resulting

curve.

Analyzing Figure 3 shows that three sections can be distin-

guished:

•

At higher power levels the error increases.

•

A middle section where the error is constant and rela-

tively small.

•

At lower power levels the error increases again.

These three sections are leading back to three error mecha-

nisms. At higher power levels the detectors output starts to

saturate because the output voltage approaches the maxi-

mum signal swing that the detector can handle. The maxi-

mum 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 R

FB of

Figure 1 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 contri-

bution 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 3

run parallel to each other. Since the conversion gain varia-

tion is acceptable, the focus will be on the pedestal voltage

variation over temperature.

20127884

FIGURE 2. Detection Curve

20127869

FIGURE 3. Input referred Error vs. RF Input Power

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