ADL5906 Data Sheet
Rev. A | Page 20 of 32
BASIS FOR ERROR CALCULATIONS
The slope and intercept used in the error plots are calculated using
the coefficients of a linear regression performed on data collected
in its central operating range. The error plots in the Typical
Performance Characteristics section are shown in two formats:
error from the ideal line and error with respect to the 25°C output
voltage. The error from the ideal line is the decibel difference in
VRMS from the ideal straight-line fit of VRMS calculated by the linear
regression fit over the linear range of the detector, typically at
25°C. The error in decibels is calculated by
Error (dB) = (VRMS − Slope × (PIN − PZ))/Slope (9)
where PZ is the x-axis intercept expressed in decibels relative to
1 mW (the input amplitude that produces a 0 V output if such an
output were possible).
The error from the ideal line is not a measure of absolute accuracy
because it is calculated using the slope and intercept of each device.
However, it verifies the linearity and the effect of temperature
and modulation on the response of the device. An example of
this type of plot is Figure 9. The slope and intercept that form
the ideal line are those at 25°C with CW modulation. Figure 4,
Figure 5, Figure 7, and Figure 8 show the error with various
popular forms of modulation with respect to the ideal CW
line. This method for calculating error is accurate, assuming
that each device is calibrated at room temperature.
In the second plot format, the VRMS voltage at a given input
amplitude and temperature is subtracted from the corresponding
VRMS at 25°C and then divided by the 25°C slope to obtain an error
in decibels. This type of plot does not provide any information
on the linear-in-dB performance of the device; it merely shows
the decibel equivalent of the deviation of VRMS over temperature,
given a calibration at 25°C. When calculating error from any
one particular calibration point, this error format is accurate. It
is accurate over the full range shown on the plot assuming that
enough calibration points are used. Figure 12 shows this plot type.
The error calculations for Figure 34 are similar to those for the
VRMS plots. The slope and intercept of the VTEMP function vs.
temperature are determined and applied as follows:
Error (°C) = (VTEMP − Slope × (Temp − TZ))/Slope (10)
where:
VTEMP is the voltage at the TEMP pin at that temperature.
Slope is, typically, 4.8 mV/°C.
Temp is the ambient temperature of the ADL5906 in degrees
Celsius.
TZ is the x-axis intercept expressed in degrees Celsius (the
temperature that would result in a VTEMP of 0 V if this were
possible).
MEASUREMENT MODE BASIC CONNECTIONS
The basic connections circuit for ADL5906 is shown in Figure 51.
The ADL5906 requires a single supply of nominally 5 V. The
supply is connected to the VPOS1 and VPOS2 supply pins.
Decouple each of these pins using two capacitors with values equal
or similar to those shown in Figure 51. Place these capacitors as
close as possible to the VPOS pins. The three no connect pins
(NIC) are not internally connected. Leave these pins unconnected.
An external 60.4 Ω resistor combines with the relatively high RF
input impedance of the ADL5906 to provide a broadband 50 Ω
match. Place an ac coupling capacitor between this resistor and
RFIN+. AC-couple the RFIN− input to ground using the same
value capacitor. To operate down to 10 MHz, the coupling
capacitors must be at least 100 pF.
The ADL5906 is placed in measurement mode by connecting
the VRMS pin to the VSET pin. In measurement mode, the output
voltage is proportional to the log of the rms input signal level.
SETTING VTADJ
As described in the Theory of Operation section, the output
temperature drift can be compensated by applying a voltage to
the TADJ pin. The compensating voltage varies with frequency.
The voltage for the TADJ pin can be easily derived from a resistor
divider connected to the VREF pin. Table 4 shows the recommended
VTADJ voltages for operation from −55°C to +125°C, along with
resistor divider values. Resistor values are chosen so that they
neither pull too much current from the VREF pin (IOUTMAX = 4 mA)
nor are so large that the maximum bias current at a VTADJ = 1 V
(14 µA) affects the resulting voltage.
The VTADJ function provides temperature compensation of
theoutput slope of the ADL5906. The Using VTEMP to Improve
Intercept Temperature Drift section describes how the temperature
stability of the ADL5906 can be further improved.
Table 4. Recommended VTADJ Voltages
Frequency VTADJ (V) R9 (Ω) R12 (Ω)
10 MHz to 2.14 GHz 0.35 1500 270
2.6 GHz 0.4 1500 316
3.5 GHz 0.45 1500 365
5.8 GHz 1.0 1540 1200
8 GHz 1.0 1540 1200
10 GHz 1.0 1540 1200