Datasheet ADL5904 (Analog Devices) - 19

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Data Sheet ADL5904 CHOOSING A VALUE FOR C 100k 700 RMS
C
10% TO 90% RISE TIME (µs)
RMS provides the averaging function for the internal rms
10k 90% TO 10% FALL TIME (µs) 600
computation. Using the minimum value for C
OUTPUT NOISE (mV p-p)
RMS allows the
)
quickest response time to a pulsed waveform, but leaves signifi-
s 1k 500 p) µ p- (
cant output noise on the output voltage signal. However, a large
V S m E (
filter capacitor reduces output noise and improves the rms
100 400 E
measurement accuracy but at the expense of the response time.
OIS LL TIM 10 300 T N /FA
In applications where the response time is not critical, place a
E U IS TP R
relatively large capacitor on the CRMS pin. In Figure 44, a value
1.0 200 OU
of 100 nF is used. For most signal modulation schemes, this value
0.1 100
ensures excellent rms measurement accuracy and low residual output noise. There is no maximum capacitance limit for CRMS.
0 0 0.1 1 10 100 1000
047 Figure 45 and Figure 46 show how output noise varies with
CRMS CAPACITANCE (nF)
13838- CRMS when the ADL5904 is driven by a single-carrier W-CDMA Figure 46. Output Noise, Rise and Fall Times vs. CRMS Capacitance, (Test Model TM1-64, peak envelope power = 10.6 dB, bandwidth Single-Carrier LTE (Test Model TM1-20) at 900 MHz with PIN = 0 dBm = 3.84 MHz) and by an LTE signal (Test Model TM1-20, peak Table 5 shows the recommended minimum values of C envelope power = 11.58 dB, bandwidth = 20 MHz), respectively. RMS for various modulation schemes. Table 5 also shows the output rise Figure 45 and Figure 46 also show how the value of CRMS affects and fal times and noise performance. Using lower capacitor the response time. This response time is measured by applying values results in faster response times but can result in degraded an RF burst at 2.14 GHz at 0 dBm to the ADL5904. The 10% to rms measurement accuracy. If the output noise shown in Table 5 90% rise time and 90% to 10% fal time are then measured. is unacceptably high, it can be reduced by increasing CRMS or by
100k 600
implementing an averaging algorithm after the output voltage of
10% TO 90% RISE TIME (µs)
the ADL5904 is sampled by an analog-to-digital converter (ADC).
90% TO 10% FALL TIME (µs) 10k 500 OUTPUT NOISE (mV p-p)
The values in Table 5 were experimental y determined to be the
) p) s
minimum capacitance that ensures good rms accuracy for that
µ ( 1k 400 p- V S
particular signal type. This test was initial y performed with a
m E ( E
large capacitance value on the CRMS pin (for example, 10 µF).
100 300 OIS LL TIM
The value of VRMS was noted for a fixed input level (for example,
T N /FA
−10 dBm). The value of C
E U
RMS was then progressively reduced (this
IS 10 200 TP R
can be accomplished with press-down capacitors) until the value
OU
of VRMS started to deviate from its original value (this indicates
1.0 100
that the accuracy of the rms computation is degrading and that CRMS is becoming too small).
0.1 0 0.1 1 10 100 1000
046 In general, the minimum CRMS value required increases as the
CRMS CAPACITANCE (nF)
peak to average ratio of the carrier increases. The minimum 13838- Figure 45. Output Noise, Rise and Fall Times vs. CRMS Capacitance, required CRMS also tends to increase as the bandwidth of the Single-Carrier W-CDMA (Test Model TM1-64) at 900 MHz with PIN = 0 dBm carrier decreases. With narrow-band carriers, the noise spectrum of the VRMS output tends to have a correspondingly narrow profile. The relatively narrow spectral profile requires a larger value of CRMS that reduces the low-pass corner frequency of the averaging function and ensures a valid rms computation.
Table 5. Recommended Minimum CRMS Values for Various Modulation Schemes Peak Envelope Power Ratio Ratio Carrier Output Noise Rise/Fall Modulation/Standard (dB) Bandwidth (MHz) CRMSMIN (nF) (mV p-p) Times (µs)
QPSK, 5 MSPS (SQR COS) Filter, α = 0.35) 3.3 5 10 42 4/25 QPSK ,15 MSPS (SQR COS Filter, α = 0.35) 3.3 15 1 38 0.5/6 64 QAM, 1 MSPS (SQR COS Filter, α = 0.35) 7.4 1 100 64 35/276 64 QAM, 5 MSPS (SQR COS Filter, α = 0.35) 7.4 5 100 54 35/276 64 QAM, 13 MSPS (SQR COS Filter, α = 0.35) 7.4 13 10 56 4/25 W-CDMA, One-Carrier, TM1-64 10.6 3.84 100 92 35/276 W-CDMA Four-Carrier, TM1-64, TM1-32, TM1-16, TM1-8 15.96 18.84 100 98 35/276 LTE, TM1, One-Carrier, 20 MHz (2048 QPSK Subcarriers) 11.58 20 100 80 35/276 Rev. B | Page 19 of 27 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS TEST CIRCUITS THEORY OF OPERATION BASIC CONNECTIONS FOR RMS MEASUREMENT CHOOSING A VALUE FOR CRMS VRMS CALIBRATION AND ERROR CALCULATION BASIC CONNECTIONS FOR THRESHOLD DETECTION Q AND QB RESPONSE TIME SETTING THE VIN− THRESHOLD DETECTION VOLTAGE APPLICATIONS INFORMATION EVALUATION BOARD SCHEMATIC AND CONFIGURATION OPTIONS OUTLINE DIMENSIONS ORDERING GUIDE