RFB

ADC

-

+

FB

OUT

+

LMV232

VREF

IOS

IDET

-

+

LMV232

SNWS017C – DECEMBER 2004 – REVISED MARCH 2013

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The advantage of this strategy is that calibration is required only once during manufacturing and not during

normal operation. The disadvantage is the fact that this method neither compensates for the residual

temperature drift of the reference voltage VREF nor for offset current variations. Only part-to-part variations at

room temperature are eliminated by this strategy. Especially the residual temperature drift negatively affects the

measurement accuracy.

Strategy 2: Elimination of Temperature Spread in VREF

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 VREF. One pin of the ADC is connected

to FB and one is connected to OUT (Figure 26).

Figure 26. Strategy 2: Differential Measurement

The power measurement is independent of the reference voltage VREF, since the ADC reading is:

VOUT-VFB = (IOS + IDET) * RFB

The reading of the ADC obviously doesn’t contain the reference voltage source VREF 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 eliminated in this strategy.

Strategy 3: Complete Elimination of Temperature Spread in Pedestal Voltage

The most accurate measurement is strategy 3, which eliminates the temperature variation of both the reference

voltage VREF 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 measurement, 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 compensates both for the reference voltage VREF as well as for the offset current

variation IOS. The frequency of the ‘calibration measurement’ can be significantly lower than those of power

measurements, depending on how fast the temperature of the device changes.

12

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