LMV232TL datasheet(11/13 Pages) NSC

11 / 13 page

Application Notes (Continued)

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 V

REF.

The other is an offset current I

OS that is generated inside the

detector. This depicted in Figure 4. Depending on the mea-

surement strategy one or both error sources can be elimi-

nated.

The error sources of the pedestal voltage can be shown in a

formula for V

OUT:

OUT =VREF +(IOS +IDET)*RFB

Where I

DET represents the intended detector output signal.

In the absence of RF input power I

DET equals zero. The

formula for the pedestal voltage can therefore be written as:

PEDESTAL =VREF +IOS *RFB

For low input power levels, the pedestal variation V

PEDESTAL

is the dominant cause of error. Besides temperature varia-

tion of the pedestal voltage, which limits the lower end of the

range, the pedestal voltage can also vary from part-to-part.

By applying a suitable measurement strategy, the pedestal

voltage error contribution can be significantly reduced or

eliminated completely.

POWER MEASUREMENT STRATEGIES

This section describes the measurement strategies to re-

duce or eliminate the pedestal voltage variation. Which strat-

egy is chosen depends on the possibilities for a factory trim

and implementation of calibration procedures.

Since the pedestal voltage is the reference level for the

LMV232, it needs to be calibrated/measured at least once to

eliminate part-to-part spread. This is required to determine

the exact detector output signal. Because of process toler-

ances, the absolute part-to-part variation of the output volt-

age in the absence of RF input power will be in the order of

5 - 10%. All measurement strategies discussed eliminate this

part-to-part spread.

Strategy 1: Elimination of Part-to-Part Spread at Room

Temperature Only

In this strategy, the pedestal voltage is determined once

during manufacturing and stored into the memory of the

phone. At each power measurement this stored pedestal

level is digitally subtracted from the measured output signal

of the LMV232 during normal operation. The procedure is

thus:

•

Measure the detector output in the absence of RF power

during manufacturing.

•

Store the output voltage value in the cell phone memory

(after it is analog-to-digital converted).

•

Subtract the stored value from each detector output read-

ing.

The advantage of this strategy is that calibration is required

only once during manufacturing and not during normal op-

eration. The disadvantage is the fact that this method neither

compensates for the residual temperature drift of the refer-

ence voltage V

REF nor for offset current variations. Only

part-to-part variations at room temperature are eliminated by

this strategy. Especially the residual temperature drift nega-

tively affects the measurement accuracy.

Strategy 2: Elimination of Temperature Spread in V

REF

If software changes need to be reduced to a minimum and

the baseband chip has a differential ADC, strategy 2 can be

used to eliminate temperature variations of the reference

voltage V

REF. One pin of the ADC is connected to FB and

one is connected to OUT (Figure 6).

The power measurement is independent of the reference

voltage V

REF, since the ADC reading is:

OUT-VFB =(IOS +IDET)*RFB

20127805

FIGURE 4. Pedestal Voltage

20127806

FIGURE 5. Strategy 1: Room Temperature Calibration

20127807

FIGURE 6. Strategy 2: Differential Measurement

LMV232TL Datasheet(PDF) 11 Page - National Semiconductor (TI)

LMV232TL Datasheet(HTML) 11 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13

Datasheet Download

Link URL