AD8362
If optimized for use at lower frequencies, CFLT should be
increased accordingly; for audio applications, use 0.1 µF. In RF
measurements where the carrier frequency is known, the
coupling and bandwidth limiting between the ICs might be
provided by a narrow-band SAW filter. Figure 62 shows the
output and law conformance error for this AD8330/AD8362
collaboration. The dynamic range extends from 5 µV to 0.5 V
rms between the 0.5 dB error points in this simulation.
For example, a 10:1 change of VTGT from 0.35 V to 3.5 V shifts
the intercept by 20 dB. This has the effect of stretching the
measurement range by the same amount, from >60 dB to more
than 80 dB. So the slope decreases to about 40 mV/dB because a
larger input range is now represented by the same 3.15 V. The
simulation results shown in Figure 64 compare the expanded
range response with that for a fixed VTGT. The upper end of
the measurement range is extended from 1.5 V to over 4 V
(limited by the input protection).
4
3
2
1
4
3
V
= V
OUT
TGT
V
= 1.25V
2
1
TGT
0
3
2
0
15
1
0
10
5
V
= V
OUT
TGT
–1
–2
–3
V
= 1.25V
TGT
0
500m
5µ
50µ
500µ
5m
50m
RMS INPUT VOLTAGE (V)
100µ
1m
10m
0.1
1
10
RMS INPUT VOLTAGE (V)
Figure 62. Output and Conformance for the AD8330/AD8362 Collaboration
Figure 64. Dynamic Range Expansion Using VTGT = VOUT
RF POWER METER WITH 80 DB RANGE
However, it is apparent that the transfer function is no
longer a simple logarithmic law; further consideration shows
that the modified function is nonanalytic. Nevertheless, this
function is dependable, and it remains as stable over supply
and temperature variations as in the normal mode. A good
approximation is provided by
According to simulations, the basic 60 dB measurement range
of the AD8362 can be extended by up to 20 dB by using a target
voltage, VTGT, that increases progressively with the input level.
In the simplest case, this can be achieved by connecting VTGT
to the output VOUT/VSET. Figure 63 shows the connections;
for present purposes, R1 is omitted and R2 is short-circuited.
3
VOUT = VSLP
log10
VIN VZ
−11.3m
log10
VIN
)
(16)
For small signal inputs, VOUT is also small, and the target is
well below the normal 75 mV (with 1.25 V applied to VTGT).
The lower target means that the AD8362’s VGA output does
not have to be as large as normal, which increases the input
sensitivity. As the signal and thus VOUT increases, so does the
target voltage, which progressively shifts the required VGA
input to a higher level.
′
where the modified slope voltage VSLP'' is 0.868 V, that is,
43.4 mV/dB. Using this expression, the dynamic range is 86 dB
to the 0.5 dB error points (0.2 mV ≤ VIN ≤ 4 V). The actual
range is reduced in practice by the effects of the AD8362’s
input-referred noise at low inputs. If the basic 60 dB+ range is
only slightly less than required in a particular application, then
a fraction of VOUT can be summed with a part of VREF to the
VTGT pin, which is why R1 and R2 were included. The output
now conforms in general terms to the formula
AD8362
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
COMM ACOM
CHPF
DECL
INHI
VREF
VTGT
VPOS
VOUT
VSET
R1
R2
3
VOUT =VSLP
log10
VIN VZ
− KC
log10
(
VIN
)
(17)
′
′
V
INLO
DECL
OUT
where the correction factor KC introduces the required
PWDN ACOM
COMM CLPF
nonlinear correction to minimize the law-conformance error.
Table 5 provides several representative spot values using
progressively greater amounts of dynamic range extension.
Figure 63. RF Power Meter with 80 dB Range
Rev. B | Page 30 of 36
ADI [ ADI ]
