CN-0150

Circuit Note

Rev. C | Page 2 of 5

CIRCUIT DESCRIPTION

The RF signal being measured is applied to the AD8318. The

device is configured in its so-called measurement mode, with

the VSET and VOUT pins connected together. In this mode,

the output voltage vs. the input signal level is linear-in-dB

(nominally −24 mV/dB) and has a typical output voltage range

of 0.5 V to 2.1 V.

The AD8318 output is connected directly to the AD7887, 12-bit

ADC. The ADC uses its internal reference and is configured for

a 0 V to 2.5 V input, resulting in an LSB size of 610 μV. With the RF

detector providing a nominal −24 mV/dB, the digital resolution

is 39.3 LSBs/dB. With this much resolution, there is little value

in trying to scale the 0.5 V to 2.1 V signal from the RF detector

to exactly fit the 0 V to 2.5 V range of the ADC.

The transfer function of the detector can be approximated by

the equation

where SLOPE is in mV/dB (−24 mV/dB nominal); INTERCEPT is

the x-axis intercept with a unit of dBm (20 dBm nominal);

of detector output voltage vs. input power is shown in Figure 2.

2.4

0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.0

1.5

1.0

0.5

0

–0.5

–1.0

–1.5

–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 0

15

10

5

INTERCEPT

ERROR 25°C

RANGE OF

CALCULATION

OF SLOPE AND

INTERCEPT

Figure 2. Typical Output Voltage vs. Input Signal Level for the AD8318

At the output of the ADC, the equation can be written as

CODE_OUT = SLOPE_ADC × (PIN − INTERCEPT)

where SLOPE_ADC is in codes/dB and PIN and INTERCEPT

are in dBm. Figure 3 shows a typical detector power sweep in

terms of input power and observed ADC codes.

Because the slope and intercept of the system vary from device

to device, a system level calibration is required. A calibration is

performed by applying two known signal levels close to the

endpoints of the AD8318 linear input range and measuring the

corresponding output codes from the ADC. The calibration

points chosen should be well within the linear operating

range of the device (−10 dBm and −50 dBm in this case).

Using the two known input power levels, PIN_1 and PIN_2,

and the corresponding observed ADC codes, CODE_1 and

CODE_2, SLOPE_ADC, and INTERCEPT can be calculated

using the following equations:

SLOPE_ADC = (CODE_2 − CODE_1)/(PIN_2 − PIN_1)

INTERCEPT = PIN_2 − (CODE_2/SLOPE_ADC)

Once SLOPE_ADC and INTERCEPT are calculated and stored (in

nonvolatile RAM) during factory calibration, they can be used

to calculate an unknown input power level, PIN, when the

equipment is in operation in the field using the equation

PIN = (CODE_OUT/SLOPE_ADC) + INTERCEPT

Figure 3 through Figure 8 show how the system transfer function

deviates from this straight line equation, particularly at the

endpoints of the transfer function. This deviation is expressed

in dB using the equation

Error (dB) = Measured Input Power − True Input Power =

(CODE_OUT/SLOPE_ADC) + INTERCEPT – PIN_TRUE

where:

CODE_OUT is the ADC output code.

SLOPE_ADC is the stored ADC slope in codes/dB.

INTERCEPT is the stored intercept.

PIN_TRUE is the exact (and unknown) input level.

The plots shown in Figure 3 through Figure 8 show the typical

system performance that can be obtained using the AD8318 and

AD7887BR in an RF power measurement system. The graphs

depict the RF input power in dBm vs. the ADC output code and

output error in dB (scaled on the axes on the right side of the

plots). They were generated from data taken with various input

power levels, frequencies, and temperatures and with both internal

and external ADC voltage references. The charts show improved

system performance and lower temperature drift with the use of

a low drift external ADC voltage reference. (See the Common

Variations section for more details about the use of an external

reference.

A complete design support package for this circuit note can be

found at www.analog.com/CN0150-DesignSupport.

4.0k

3.5k

3.0k

2.5k

2.0k

1.5k

1.0k

0.5k

4

3

2

1

0

–2

–1

–3

–4

0

–70

–60 –50

–40

–30

–20

10

–10

0

INPUT POWER (dBm)

CODE_2

PIN_2

PIN_1

CODE_1

+25°C CODE

–40°C CODE

+85°C CODE

+25°C ERROR

–40°C ERROR

+85°C ERROR

Figure 3. Input = 900 MHz, ADC Using an Internal 2.5 V Reference