CN-0187

Circuit Note

Rev. 0| Page 4 of 7

1ms/DIV

70mV rms

160mV rms

250mV rms

400mV rms RF INPUT

VRMS

PULSED RFIN

Figure 7. Output Response to Various RF Input Pulse Levels, Supply 3 V,

900 MHz Frequency, Square-Domain Filter Open, Output Filter 0.1 μF

with Parallel 1 kΩ

The RMS and PEAK outputs of the ADL5502 pass through

unity gain buffers that drive cross-coupled stages for converting

the single-ended outputs to differential signals. The internal

passes through another unity gain buffer and a voltage divider.

This sets the common-mode voltage of the network to +1.25 V.

The AD7266 achieves simultaneous samples of the RMS and

PEAK outputs and transfers the data within a 1 µs response

time. The data is provided on a single serial data line. Because

slope and intercept vary from device to device, board-level

calibration must be performed to achieve high accuracy. In

general, calibration is performed by applying two input power

levels to the ADL5502 and measuring the corresponding output

voltages. The calibration points are generally chosen to be

within the linear operating range of the device. The best-fit line

is characterized by calculating the conversion gain (or slope)

and intercept using the following equations:

(1)

(2)

where:

Once gain and intercept are calculated, an equation can be

written that allows calculation of an (unknown) input power

based on the measured output voltage.

(3)

For an ideal (known) input power, the law conformance error of

the measured data can be calculated as

×

–

log

×

20

(dB)

















=

IN, IDEAL

URED

VRMS, MEAS

V

Gain

Intercept

V

ERROR

(4)

Figure 8 and Figure 9 show plots of the VRMS and PEAK error

at 25°C, the temperature at which the ADL5502 is calibrated.

Note that the error is not zero; this is because the ADL5502

does not perfectly follow the ideal linear equation, even within

its operating region. The error at the calibration points is,

however, equal to zero by definition.

–3

–2

–1

0

1

2

3

INPUT (dBm)

–25

–20

–15

–10

–5

0

5

10

15

450MHz

900MHz

1900MHz

2350MHz

2600MHz

Figure 8. Measured VRMS Linearity Error vs. Input Level, 450 MHz, 900 MHz,

1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V

–3

–2

–1

0

1

2

3

INPUT (dBm)

–25

–20

–15

–10

–5

0

5

10

15

450MHz

900MHz

1900MHz

2350MHz

2600MHz

Figure 9. Measured PEAK Linearity Error vs. Input Level, 450 MHz, 900 MHz,

1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V

When the characteristics (slope and intercept) of the VRMS and

PEAK outputs are known, the calibration for the CF calculation

is complete. A three-stage process must be taken to measure

and calculate the crest factor of any waveform. First, the

unknown signal must be applied to the RF input, and the

corresponding VRMS level is measured. This level is indicated