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RMS Output Interface and Transient Response
Output transient response is determined by the integration capacitance (CINT), and output load conditions. Using
larger values of CINT will narrow the operating bandwidth of the integrator, resulting in a longer averaging time-interval
and a more filtered output signal; however it will also slow the power detector’s transient response. A larger CINT value
favors output accuracy over speed. For the fastest possible transient settling times, leave the CINT pin free of any
external capacitance. This configuration will operate the integrator at its widest possible bandwidth, resulting in short
averaging time-interval and an output signal with little filtering. Most applications will choose to have some external
integration capacitance, maintaining a balance between speed and accuracy. Furthermore, error performance over
crest factor is degraded when CINT is very small (for CINT<100pF).
Modulation and deviation results in Electrical Specification Table 2 are given for CINT = 0.1 μF.
Start by selecting CINT using the following expression, and then adjust the value as needed, based on the application’s
preference for faster transient settling or output accuracy.
CINT = 1500 μF / (2*π*LAM), in Farads, where LAM= lowest amplitude-modulation component frequency in Hertz.
Example: when LAM= 10 kHz, CINT = 1500μF/(2*π*10kHz) = 24nF ~ 22nF
Input signal is 1900MHz CW-tone switched on and off
RMS is loaded with 1kΩ, 4pF, and VTGT = VREF,
D.R. is input dynamic range to ±1 dB error
Transient response can also be slewed by the RMS output if it is excessively loaded: keep load resistance above
375Ω. An optimal load resistance of approximately 500Ω to 1kΩ will allow the output to move as quickly as it is able.
For increased load drive capability, consider a buffer amplifier on the RMS output.
Using an integrating amplifier on the RMS output allows for an alternative treatment for faster settling times. An external
amplifier optimized for transient settling can also provide additional RMS filtering, when operating HMC610LP4 with
a lower CINT capacitance value.
Table: Transient Response vs. CINT & COFS Capacitance:
C
INT
C
OFS
RMSOUT Rise-Time
Over Dynamic Range
Pin = -26 dBm
RMSOUT Fall-Time
Pin = -11 dBm
Pin = 4 dBm
0
0 pF
15 ns
0.2 μs
0.3 μs
1.4 μs
82 pF
0 pF
65 ns
0.5 μs
0.7 μs
1.4 μs
820 pF
0 pF
0.83 μs
5 μs
8 μs
10 μs
10 nF
0 pF
3.2 μs
20 μs
30 μs
36 μs
100 nF
0 pF
69 μs
400 μs
580 μs
775 μs
0
1 nF
15 ns
1.0 μs
17 μs
28 μs
82 pF
1 nF
65 ns
1.2 μs
17 μs
28 μs
820 pF
1 nF
0.83 μs
6 μs
17 μs
28 μs
10 nF
1 nF
3.2 μs
60 μs
80 μs
105 μs
100 nF
1 nF
69 μs
500 μs
700 μs
900 μs
HMC610LP4 / 610LP4E
v12.0309
RMS POWER DETECTOR
75 dB, DC - 3.9 GHz