ADL5902-EVALZ AD [Analog Devices], ADL5902-EVALZ Datasheet - Page 20

• Manufacturer Part Number: ADL5902-EVALZ
• Description: 50 MHz to 9 GHz 65 dB TruPwr Detector
• Manufacturer: AD [Analog Devices]

ADL5902
SETTING V
As discussed in the Theory of Operation section, the output
temperature drift must 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 5 shows the recom-
mended V
resistor divider values. Resistor values are chosen so that they
neither pull too much current from VREF (VREF short-circuit
current is 4 mA) nor are so large that the TADJ pin’s bias current of
3 µA affects the resulting voltage at the TADJ pin.
Table 5. Recommended V
Frequency
100 MHz
700 MHz to 2.14 GHz
2.6 GHz
3.5 GHz
5.8 GHz
SETTING V
As discussed in the Theory of Operation section, setting the
voltage on VTGT to 0.8 V represents a compromise between
achieving excellent rms compliance and maximizing dynamic
range. The voltage on VTGT can be derived from the VREF pin
using a resistor divider as shown Figure 45. Like the resistors
chosen to set the V
have reasonable values that do not pull too much current from
VREF or cause bias current errors. Also, attention should be
paid to the combined current that VREF must deliver to
generate the V
kept well below the VREF short-circuit current of 4 mA.
CHOOSING A VALUE FOR C
C
computation. Using the minimum value for C
quickest response time to a pulsed waveform but leaves
significant output noise on the output voltage signal. By the
same token, a large filter cap reduces output noise but at the
expense of response time.
For non response-time critical applications, a relatively large
capacitor can be placed on the CLPF pin. In Figure 45, a value
of 10 µF is used. For most signal modulation schemes, this value
ensures excellent rms measurement compliance and low
residual output noise. There is no maximum capacitance limit
for C
LPF
provides the averaging function for the internal rms
LPF
.
TADJ
TADJ
TGT
for operation from −40°C to +85°C, along with
TADJ
and V
TADJ
voltage, the resistors setting V
TGT
V
0.5
0.4
0.45
0.5
0.95
TADJ
voltages. This current should be
TADJ
for Selected Frequencies
(V)
LPF
R9 (Ω)
1430
1430
1430
1430
1430
LPF
allows the
R12 (Ω)
402
301
348
402
1007
TGT
should
Rev. 0 | Page 20 of 28
Figure 46 shows how output noise varies with C
ADL5902 is driven by a single-carrier W-CDMA signal (Test
Model TM1-64, peak envelope power = 10.56 dB, bandwidth =
3.84 MHz). With a 10 µF capacitor on CLPF, there is residual
noise on V
(assuming a slope of approximately 53 mV/dB).
Figure 46 also shows how the response time is affected by the
value of C
−10 dBm was applied to the ADL5902. The 10% to 90% rise
time and 90% to 10% fall time were then measured. It is notable
that the fall time is much longer than the rise time. This can
also be seen in the response time plots, Figure 22, Figure 23,
Figure 25, and Figure 26.
In applications where the response time is critical, a different
approach to signal filtering can be taken. This is shown in
Figure 47. The capacitor on the CLPF pin is set to the minimum
value that ensures that a valid rms computation has been
performed. The job of noise removal is then handed off to an
RC filter on the VOUT pin. This approach ensures that there is
enough averaging to ensure good rms compliance and does not
burden the rms computation loop with extra filtering that will
significantly slow down the response time. By finishing the
filtering process using an RC filter after VOUT, faster fall times
can be achieved with an equivalent amount of output noise. It
should be noted that the RC filter can also be implemented in
the digital domain after the analog-to-digital converter.
Figure 46. Output Noise, Rise and Fall Times vs. C
300
250
200
150
100
50
0
1
Carrier W-CDMA (TM1-64) at 2.14 GHz with P
LPF
OUT
. To measure this, a RF burst at 2.14 GHz at
of 4.4 mV p-p, which is less than 0.1 dB error
10
C
LPF
(nF)
OUTPUT NOISE (mV p-p)
10% TO 90% RISE TIME (µs)
90% TO 10% FALL TIME (µs)
100
LPF
Capacitance, Single-
IN
LPF
= 0 dBm
when the
1000
1M
100k
10k
1k
100
10
1