AD8319_07 Datasheet, PDF (12/20 Pages) Analog Devices – 1 MHz to 10 GHz, 40 dB Log Detector/Controller

AD8319
The slope is given by −ID × 2x × 1.5 kΩ = −22 mV/dB × x. For
example, if a resistor divider to ground is used to generate a VSET
voltage of VOUT/2, x = 2. The slope is set to −880 mV/decade or
−44 mV/dB.
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, so provision is made for optimization of internal
temperature compensation at a given frequency by providing
Pin TADJ.
V_INTERNAL
AD8319
I_COMP
TADJ
R_TADJ
1.5kΩ
COMM
COMM
Figure 26. TADJ Interface
RTADJ is connected between this pin and ground. The value of
this resistor partially determines the magnitude of an analog
correction coefficient, which is used to reduce intercept drift.
The relationship between output temperature drift and
frequency is not linear and cannot be easily modeled. As a
result, experimentation is required to choose the correct TADJ
resistor. Table 4 shows the recommended values for some
commonly used frequencies.
Table 4. Recommended RTADJ Resistor Values
Frequency
Recommended RTADJ
50 MHz
18 kΩ
100 MHz
18 kΩ
900 MHz
18 kΩ
1.8 GHz
8 kΩ
1.9 GHz
8 kΩ
2.2 GHz
8 kΩ
3.6 GHz
8 kΩ
5.3 GHZ
500 Ω
5.8 GHz
500 Ω
8 GHz
Open
MEASUREMENT MODE
When the VOUT voltage or a portion of the VOUT voltage is fed
back to the VSET pin, the device operates in measurement
mode. As seen in Figure 27, the AD8319 has an offset voltage,
a negative slope, and a VOUT measurement intercept at the high
end of its input signal range.
2.00
1.75
2.0
V_OUT 25°C
ERROR 25°C
1.5
1.50
1.0
1.25
0.5
1.00
0
0.75
–0.5
0.50
–1.0
RANGE FOR
0.25
CALCULATION OF
SLOPE AND INTERCEPT
–1.5
0
–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 0
P_IN (dBm)
5 10 15
INTERCEPT
Figure 27. Typical Output Voltage vs. Input Signal
The output voltage vs. input signal voltage of the AD8319 is
linear-in-dB over a multidecade range. The equation for this
function is of the form
VOUT = X × VSLOPE/DEC × log10(VIN/VINTERCEPT) =
X × VSLOPE/dB × 20 × log10(VIN/VINTERCEPT)
(3)
where:
X is the feedback factor in VSET = VOUT/X.
VSLOPE/DEC is nominally −440 mV/decade or −22 mV/dB.
VINTERCEPT is the x-axis intercept of the linear-in-dB portion of
the VOUT vs. VPIN curve (see Figure 27).
VINTERCEPT is 15 dBm (2 dBV) for a sinusoidal input signal.
An offset voltage, VOFFSET, of 0.35 V is internally added to the
detector signal, so that the minimum value for VOUT is
X × VOFFSET, so for X = 1, minimum VOUT is 0.35 V.
The slope is very stable vs. process and temperature variation.
When base-10 logarithms are used, VSLOPE/DEC represents the
volts/decade. A decade corresponds to 20 dB; VSLOPE/DEC/20 =
VSLOPE/dBB represents the slope in volts/dB.
As noted in the Equation 1 and Equation 2, the VOUT voltage has
a negative slope. This is also the correct slope polarity to control
the gain of many power amplifiers in a negative feedback configu-
ration. Because both the slope and intercept vary slightly with
frequency, it is recommended to refer to the Specifications
section for application-specific values for the slope and intercept.
Although demodulating log amps respond to input signal
voltage, not input signal power, it is customary to discuss the
amplitude of high frequency signals in terms of power. In this
case, the characteristic impedance of the system, Z0, must be
known to convert voltages to their corresponding power levels.
Equation 4 to Equation 6 are used to perform this conversion.
P(dBm) = 10 × log10(Vrms²/(Z0 × 1 mW))
(4)
P(dBV) = 20 × log10(Vrms/1 Vrms)
(5)
P(dBm) = P(dBV) − 10 × log10(Z0 × 1 mW/1 Vrms²)
(6)
Rev. A | Page 12 of 20