ADL5513ACPZ-R2 datasheet(17/28 Pages) AD

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ADL5513

Rev. 0 | Page 17 of 28

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–70 –65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

PIN (dBm)

+25°C

–40°C

+85°C

+125°C

Figure 33. Typical Device at 3600 MHz, Calibration Points at PIN = −12 dBm

and −40 dBm

In Figure 32, calibration points are chosen so that linearity is

improved over the full dynamic range, but error at the higher

power level at PIN = −10 dBm is 0.5 dB at 25°C. In Figure 33,

calibration points are chosen so that error is smaller at higher

power input ,but with loss of linearity over the full dynamic range.

Figure 34 shows another way of presenting the error of a log

amp detector. The same typical device from Figure 32 and

Figure 33 is presented where the error at −40°C, +85°C, and

+125°C are calculated with respect to the output voltage at

+25°C. This is the key difference in presenting the error of a log

amp compared with the plots in Figure 32 and Figure 33 where

the error is calculated with respect to the ideal line at 25°C.

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–70 –65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

PIN (dBm)

–40°C

+85°C

+125°C

Figure 34. Error vs. Temperature with Respect to Output Voltage at 25°C,

3600 MHz

With this alternative technique, the error at ambient becomes

0 dB by definition. This would be valid if the device transfer

function perfectly followed the ideal equation or if there were

many calibration points used.

VOUT = Slope × (PIN − Intercept)

Because the log amp never perfectly follows this equation, espe-

cially outside of its linear range, Figure 34 can be misleading as

a representation of log amp error. This plot tends to artificially

improve linearity and extend the dynamic range, unless enough

calibration points are used to remove error.

Figure 34 is a useful tool for estimating temperature drift at a

particular power level with respect to the (nonideal) output

voltage at ambient.

TEMPERATURE COMPENSATION OF OUTPUT

VOLTAGE

The primary component of the variation in VOUT vs. temperature as

the input signal amplitude is held constant is the drift of the

intercept. This drift is also a weak function of the input signal

frequency; therefore, a provision is made for the optimization of

the internal temperature compensation at a given frequency by

providing Pin TADJ with dual functionality. The first function

for this pin is temperature compensation and the second function

is to power down the device when VTADJ = VPOS − 0.3 V (see the

Power-Down Functionality section).

PWDN/TADJ

COMM

VINTERNAL

ICOMP

Figure 35. TADJ Interface

VTADJ is a voltage forced between TADJ and ground. The value

of this voltage determines the magnitude of an analog correction

coefficient, which is used to reduce intercept drift.

The relationship between output temperature drift and fre-

quency is not linear and cannot be easily modeled. As a result,

experimentation is required to select the optimum VTADJ voltage.

The VTADJ voltage applied to Pin TADJ can be supplied by a

DAC with sufficient resolution, or Resistor R8 and Resistor R9

on the evaluation board (see Figure 47) can be configured as a

voltage divider using VPOS as the voltage source.

Table 4 shows the recommended voltage values for some

commonly used frequencies in characterization to optimize

operation at 85°C. The TADJ pin has high input impedance.

Table 4. Recommended VTADJ Values

Frequency

Recommended V

TADJ (V)

100 MHz

0.89

900 MHz

0.86

1.9 GHz

0.80

2.14 GHz

0.84

2.6 GHz

0.83

3.6 GHz

0.90

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