AD8364-EVAL Analog Devices, AD8364-EVAL Datasheet - Page 19

• Manufacturer Part Number: AD8364-EVAL
• Description: LF to 2.7GHz, Dual 60dB TruPwr Detector
• Manufacturer: Analog Devices

PRELIMINARY TECHNICAL DATA
absolute value of compensation is different for various
frequencies. The following chart can be used to apply the
appropriate ADJ[A/B] voltage to maintain a temperature drift
error less than +/- 0.5dB over the entire temperature range.
F (MHz)
ADJ[A/B] (V)
Compensating the device for temperature drift using ADJ[A/B]
allows for great flexibility. If the user requires minimum
temperature drift at a given input power or subset of the dynamic
range, the ADJ[A,B] voltage can be swept while monitoring
OUT[A,B] over temperature. Figure 33 shows an example of
this. The ADJ[A,B] value where the output does not change is
the voltage that must be applied to have minimum temperature
drift at the given power and frequency.
The ADJ[A,B] input has a high input impedance. The input can
be conveniently driven from an attenuated value of VREF, using
a resistor divider.
ALTERING THE SLOPE
None of the changes in operating conditions discussed so far affect
the logarithmic slope, V
be altered by controlling the fraction of VOUT that is fed back to the
setpoint interface at the VSET pin. When the full signal from VOUT
is applied to VSET, the slope assumes its nominal value of 50 mV/dB.
It can be increased by including an attenuator between these pins, as
shown in Figure 34. Moderately low resistance values should be used
to minimize scaling errors due to the 70 kΩ input resistance at the
VSET pin. Keep in mind that this resistor string also loads the
output, and it eventually reduces the load-driving capabilities if very
low values are used. To calculate the resistor values, use
where S
R2' is the value of R2 in parallel with 70 kΩ. For example, using R1 =
1.65 kΩ and R2 = 1.69 kΩ (R2' = 1.649 kΩ), the nominal slope is
1.65
1.55
1.45
1.7
1.6
1.5
1.4
0
R1
D
=
Figure 33. OUTA vs. ADJA over Temp. Pin=-30dBm, 1.9GHz
is the desired slope, expressed in mV/dB, and
0.25
R2
'
(
450
S
0
D
0.5
50 −
900
0.75
0
1
SLP
)
, in Equation 9. However, this can readily
1700
1
0
Table 4
ADJA (V)
1.25
1900
0.75
1.5
2200
1.02
1.75
85C
65C
45C
25C
10C
-20C
-40C
2500
1.14
2
2700
1.18
2.25
(17)
2.5
Rev. PrC Ι Page 19 of 23
increased to 100 mV/dB. This choice of scaling is useful when
the output is applied to a digital voltmeter because the displayed
number reads as a decibel quantity directly, with only a decimal
point shift.
Operation at high slopes is useful when a particular sub-range
of the input is measured in greater detail. However, a
measurement range of 60 dB would correspond to a 6 V change
in VOUT at this slope, exceeding the capacity of the AD8364’s
output stage when operating on a 5 V supply. This requires that
the intercept is repositioned to place the desired sub-range
within a window corresponding to an output range of 0.2 V ≤
VOUT ≤ 4.8 V,
a 46 dB range.
Figure 35, an output of 0.5 V corresponds to the lower end of
the desired sub-range, and 4.5 V corresponds to the upper limit
with 3 dB of margin at each end of the range, which is
nominally 3 mV rms to 300 mV rms, with the intercept at 1.9
mV rms. Note that R2 is connected to VREF rather than
ground. R3 is needed to ensure that the AD8364’s reference
buffer, which can sink only a small current, is correctly loaded.
It is apparent that a variable attenuation factor based on this
scheme could provide a manual adjustment of the slope, but
there are few situations in which this is of value. When the slope
is raised by some factor, the loop capacitor, CLPF, should be
raised by the same factor to ensure stability and to preserve a
chosen averaging time. The slope can be lowered by placing a
two-resistor attenuator after the output pin, following standard
practice.
PW
V
CO
VP
PSA
IN
IN
IN
IN
DN
SB
HA
MR
HB
LA
LB
PW
V
CO
VPSB
INHB
PSA
INHA
INLA
INLB
DN
MR
2
2
3
2
2
3
3
25
7
8
6
9
0
1
2
2
2
2
2
3
3
3
25
6
7
8
9
2
0
1
CHPB
CHPA
Figure 34. External Network to Raise Slope
24
CHPB
1
CHPA
TruPwr™
TruPwr™
DECB
Channel B
Channel A
DECA
24
1
TruPwr™
23
TruPwr™
2
DECB
Using the arrangement shown in
Channel A
Channel B
DECA
COMB ADJB ADJA VREF VLVL CLPB
23
COMA VPSR ACMB T EMP ACMA CLPA
2
22
3
COMB ADJB ADJA VREF VLVL CLPB
COMA VPSR ACMB TEMP ACMA CLPA
OUTA
OUTB
22
3
BIAS
OUTA
OUTB
21
4
BIAS
21
4
20
5
V2I
20
5
TEMP
19
6
V2I
TEMP
19
6
18
7
V2I
18
7
V2I
17
8
17
8
16
15
14
13
12
11
10
AD8364
9
16
15
14
13
12
11
10
9
VSTA
OUTA
FBKA
OUTP
OUTN
FBKB
OUTB
VSTB
VSTA
OUTA
FBKA
OUTP
OUTN
FBKB
OUTB
VSTB
R1
R2
R1
4.02K Ohms
R2
4.32K Ohms
R3
2K Ohms
Vout
Vout