LMV232TLX datasheet(12/13 Pages) NSC

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Application Notes (Continued)

The reading of the ADC obviously doesn’t contain the refer-

ence voltage source V

REF anymore, but the contribution of

the offset current remains present. This measurement is

performed during normal operation. Therefore, it eliminates

voltage reference variations over temperatures, as opposed

to strategy 1. Also offset variations in the op amp are elimi-

nated in this strategy.

Strategy 3: Complete Elimination of Temperature

Spread in Pedestal Voltage

The most accurate measurement is strategy 3, which elimi-

nates the temperature variation of both the reference voltage

REF and the offset current IOS. In this strategy, the pedestal

voltage is measured regularly during operation of the phone,

and stored in the phone memory. For each power measure-

ment, the stored value is digitally subtracted from the

(analog-to-digital converted) detector output signal. Since it

measures the pedestal voltage itself for calibration it com-

pensates both for the reference voltage V

REF as well as for

the offset current variation I

OS. The frequency of the ‘calibra-

tion measurement’ can be significantly lower than those of

power measurements, depending on how fast the tempera-

ture of the device changes.

The calibration measurement procedure can be explained

with the aid of Figure 1, which depicts a typical power

measurement setup using the LMV232. In normal operation,

the two PA’s in the setup will never be active at the same

time. One PA will produce the required transmit power, while

the other one is off, (disabled) and produces no power. The

pedestal voltage should be measured in the absence of RF

power. This can be achieved by switching the Band Select

(BS) pin such that the LMV232 input is selected where the

disabled PA is connected to. The pedestal voltage at no input

power can be read at the output pin.

Using the Band Select (BS) control pin of the LMV232:

•

Select the RF input that is connected to the disabled PA,

by the BS pin.

•

Measure the detector output.

•

Store the result in the phone memory.

•

Subtract the stored value from each detector power read-

ing, until a new update is performed.

Important advantages of this approach are that no factory

trim is required and the temperature drift of the pedestal can

be cancelled almost completely as well as the part-to-part

spread. The remaining error is determined by the resolution

of the ADC. A slight disadvantage is that on average more

than one detector reading is required per power measure-

ment. This overhead though can be made almost negligible

in normal circumstances.

20127808

FIGURE 7. Strategy 3: Calibration during normal

operation

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