Datasheet AD8314 Datasheet (Analog Devices)

100 MHz–2500 MHz 45 dB
a
FEATURES Complete RF Detector/Controller Function Typical Range –58 dBV to –13 dBV
–45 dBm to 0 dBm re 50 Frequency Response from 100 MHz to 2.5 GHz Temperature-Stable Linear-in-dB Response
Accurate to 2.5 GHz Rapid Response: 70 ns to a 10 dB Step Low Power: 12 mW at 2.7 V Power-Down to 20 ␮A
APPLICATIONS Cellular Handsets (TDMA, CDMA, GSM) RSSI and TSSI for Wireless Terminal Devices Transmitter Power Measurement and Control
PRODUCT DESCRIPTION
The AD8314 is a complete low-cost subsystem for the mea­surement and control of RF signals in the frequency range
0.1 GHz–2.5 GHz, with a typical dynamic range of 45 dB, intended for use in a wide variety of cellular handsets and other wireless devices. It provides a wider dynamic range and better accuracy than possible using discrete diode detectors. In particular, its temperature stability is excellent over the full operating range of
–30°C to +85°C.
Its high sensitivity allows control at low power levels, thus reducing the amount of power that needs to be coupled to the detector. It is essentially a voltage-responding device, with a typical signal range of 1.25 mV to 224 mV rms or –58 dBV to
–13 dBV. This is equivalent to –45 dBm to 0 dBm re 50 Ω.
RF Detector/Controller
AD8314
For convenience, the signal is internally ac-coupled, using a 5 pF
capacitor to a load of 3 k in shunt with 2 pF. This high-pass
coupling, with a corner at 16 MHz, determines the lowest oper­ating frequency. Thus, the source may be dc-grounded.
The AD8314 provides two voltage outputs. The first, called V_UP, increases from close to ground to about 1.2 V as the input signal level increases from 1.25 mV to 224 mV. This output is intended for use in measurement mode. Consult the Appli­cations section of this data sheet for information on use in this mode. A capacitor may be connected between the V_UP and FLTR pins when it is desirable to increase the time interval over which averaging of the input waveform occurs.
The second output, V_DN, is an inversion of V_UP, but with twice the slope and offset by a fixed amount. This output starts
at about 2.25 V (provided the supply voltage is ≥3.3 V) for
the minimum input and falls to a value close to ground at the maximum input. This output is intended for analog control loop applications. A setpoint voltage is applied to VSET and V_DN is then used to control a VGA or power amplifier. Here again, an external filter capacitor may be added to extend the averaging time. Consult the Applications section of this data sheet for information on use in this mode.
The AD8314 is available in a micro_SOIC package and con­sumes 4.5 mA from a 2.7 V to 5.5 V supply. When powered
down, the typical sleep current is 20 µA.
FUNCTIONAL BLOCK DIAGRAM
RFIN
OFFSET COMP'N
COMM
(PADDLE)
REV. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
FLTR
DETDETDETDETDET
10dB10dB 10dB
10dB
BAND-GAP
AD8314
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 1999
REFERENCE
V-I
I-V
X2
VPOS ENBL
VSET
V
UP
V DN
AD8314–SPECIFICATIONS
(VS = 3 V, TA = +25C, unless otherwise noted)
Parameter Condition Min Typ Max Unit
OVERALL FUNCTION
Frequency Range To Meet All Specifications 0.1 2.5 GHz Input Voltage Range Internally AC-Coupled 1.25 224 mV rms
Equivalent Power Range 52.3 External Termination –45 0 dBm
Logarithmic Slope Main Output, V_UP, 100 MHz
1
18.85 21.3 23.35 mV/dB
Logarithmic Intercept Main Output, V_UP, 100 MHz –68 –62 –56 dBV
Equivalent dBm Level 52.3 External Termination –55 –49 –43 dBm
INPUT INTERFACE (Pin RFIN)
DC Resistance to COMM 100 k Inband Input Resistance f = 0.1 GHz 3 k
Input Capacitance f = 0.1 GHz 2 pF
MAIN OUTPUT (Pin V_UP)
Voltage Range V_UP Connected to VSET 0.01 1.2 V Minimum Output Voltage No Signal at RFIN, R Maximum Output Voltage
General Limit 2.7 V ≤ V
2
R
10 k 1.9 2 V
L
5.5 V V
S
10 k 0.01 0.02 0.05 V
L
– 1.1 VS–1 V
S
Available Output Current Sourcing/Sinking 1/0.5 2/1 mA Response Time 10%–90%, 10 dB Step 70 ns
Residual RF (at 2f) f = 0.1 GHz (Worst Condition) 100 µV
INVERTED OUTPUT (Pin V_DN)
Gain Referred to V_UP V Minimum Output Voltage V Maximum Output Voltage V
= 2.25 V – 2 × V
DN
3.3 V 0.01 0.05 0.1 V
S
3.3 V
S
3
UP
2.1 2.2 2.5 V
–2
Available Output Current Sourcing/Sinking 4/100 6/200 mA/µA
Output-Referred Noise RF Input = 2 GHz, –33 dBV, f
= 10 kHz 1.05 µV/Hz
NOISE
Response Time 10%–90%, 10 dB Input Step 70 ns Full-Scale Settling Time –40 dBm to 0 dBm Input Step, to 95% 150 ns
SETPOINT INPUT (Pin VSET)
Voltage Range Corresponding to Central 40 dB 0.15 1.2 V
Input Resistance 710 k
Logarithmic Scale Factor f = 0.900 GHz 20.7 mV/dB
f = 1.900 GHz 19.7 mV/dB
ENABLE INTERFACE (Pin ENBL)
Logic Level to Enable Power HI Condition, –30°C T
Input Current when HI 2.7 V at ENBL, –30°C T
+85°C 1.6 V
A
+85°C 20 300 µA
A
POS
V
Logic Level to Disable Power LO Condition, –30°C ≤ TA +85°C –0.5 0.8 V
POWER INTERFACE (Pin VPOS)
Supply Voltage 2.7 3.0 5.5 V Quiescent Current 3.0 4.5 5.7 mA
Over Temperature –30°C ≤ T
+85°C 2.7 4.4 6.6 mA
A
Total Supply Current when Disabled 20 95 µA
Over Temperature –30°C ≤ TA ≤ +85°C40µA
NOTES
1
Mean and Standard Deviation specifications are available in Table I.
2
Increased output possible when using an attenuator between V_UP and VSET to raise the slope.
3
Refer to Figure 19 for details.
Specifications subject to change without notice.
–2–
REV. 0
AD8314
WARNING!
ESD SENSITIVE DEVICE
ABSOLUTE MAXIMUM RATINGS*
Supply Voltage VPOS . . . . . . . . . . . . . . . . . . . . . . . . . . .5.5 V
V_UP, V_DN, VSET, ENBL . . . . . . . . . . . . . . . . 0 V, VPOS
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 V rms
Equivalent Power . . . . . . . . . . . . . . . . . . . . . . . . . . . +17 dBm
Internal Power Dissipation . . . . . . . . . . . . . . . . . . . . 200 mW
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200°C
θ
JA
Maximum Junction Temperature . . . . . . . . . . . . . . . . . 125°C
Operating Temperature Range . . . . . . . . . . . –30°C to +85°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature Range (Soldering 60 sec) . . . . . . . . . 300°C
*Stresses above those listed under Absolute Maximum Ratings may cause permanent
damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating cond itions for extended periods may affect device reliability.
PIN CONFIGURATION
RFIN ENBL VSET
1
2
AD8314
TOP VIEW
3
(Not to Scale)
4
8
7
6
5
VPOS
DN
V
UP
V COMMFLTR
Pin Function Descriptions
Pin Name Function
1 RFIN RF Input. 2 ENBL Connect pin to V
for normal operation.
S
Connect pin to ground for disable mode.
3 VSET Setpoint input for operation in controller
mode. To operate in detector mode connect VSET to V_UP
4 FLTR Connection for an external capacitor to slow
the response of the output. Capacitor is con-
nected between FLTR and V_UP. 5 COMM Device Common (Ground). 6 V_UP Logarithmic output. Output voltage increases
with increasing input amplitude. 7 V_DN Inversion of V_UP, governed by the following
equation: V_DN = 2.25 V – 2 × V
UP
.
8 VPOS Positive supply voltage (VS), 2.7 V to 5.5 V.
ORDERING GUIDE
Model Temperature Range Package Description Package Option
AD8314ARM* –30°C to +85°C Tube, 8-Lead micro_SOIC RM-8
AD8314ARM-REEL 13" Tape and Reel AD8314ARM-REEL7 7" Tape and Reel AD8314-EVAL Evaluation Board
*Device branded as J5A.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD8314 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality.
REV. 0
–3–
AD8314
INPUT AMPLITUDE – dBV
4
–4
–70 0–60
ERROR – dB
–50 –40 –30 –20 –10
1
0
–1
–2
–3
2.5GHz
1.9GHz
0.9GHz
(–47dBm) (+3dBm)
0.1GHz
2
3
–Typical Performance Characteristics
1.2
1.0
0.8
0.6
– Volts
UP
V
0.4
0.2
0
–75 –5–65
1.2
1.0
0.8
0.6
– Volts
UP
V
0.4
2.5GHz
(–52dBm)
–55 –45 –35 –25 –15
INPUT AMPLITUDE – dBV
Figure 1. VUP vs. Input Amplitude
+258C
–308C
+858C
+258C
0.1GHz
1.9GHz
(–2dBm)
0.9GHz
–308C
3
2
1
0
ERROR – dB
–1
Figure 4. Log Conformance vs. Input Amplitude
1.2
1.0
0.8
0.6
– Volts
UP
V
0.4
+858C
+258C
–308C
3
2
1
0
–1
ERROR – dB
0.2
0
–60
–70 0
(–47dBm)
SLOPE AND INTERCEPT NORMALIZED AT +258C AND APPLIED TO –308C AND +858C
–50 –40 –30 –20
INPUT AMPLITUDE – dBV
–10
(+3dBm)
–2
–3
Figure 2. VUP and Log Conformance vs. Input Amplitude at
0.1 GHz; –30
– Volts V
Figure 3. VUP and Log Conformance vs. Input Amplitude at 0.9 GHz; –30
°
C, +25°C, and +85°C
1.2
1.0
0.8
0.6 –308C
UP
0.4
0.2
0
–60
–70 0
(–47dBm)
°
+258C
+858C
SLOPE AND INTERCEPT NORMALIZED AT +258C AND APPLIED TO –308C AND +858C
–50 –40 –30 –20
INPUT AMPLITUDE – dBV
C, +25°C, and +85°C
–10
(+3dBm)
3
2
1
0
ERROR – dB
–1
–2
–3
0.2
0
–70 0
(–47dBm)
SLOPE AND INTERCEPT NORMALIZED AT +258C AND APPLIED TO –308C AND +858C
–60
–50 –40 –30 –20
INPUT AMPLITUDE – dBV
–10
(+3dBm)
–2
–3
Figure 5. VUP and Log Conformance vs. Input Amplitude at 1.9 GHz; –30
1.2
1.0
0.8
0.6
– Volts
UP
V
0.4
0.2
0
°
C, +25°C, and +85°C
+858C
+858C
+258C
–308C
SLOPE AND INTERCEPT NORMALIZED AT +258C AND APPLIED TO –308C AND +858C
–60
–70 0
(–47dBm)
–50 –40 –30 –20
INPUT AMPLITUDE – dBV
–10
(+3dBm)
3
2
1
0
ERROR – dB
–1
–2
–3
Figure 6. VUP and Log Conformance vs. Input Amplitude at 2.5 GHz; –30
°
C, +25°C, and +85°C
–4–
REV. 0
AD8314
23
SLOPE – mV/dB
22
21
+258C
20
19
18
0
0.5
–308C
1.0
FREQUENCY – GHz
+858C
1.5 2.0 2.5
Figure 7. Slope vs. Frequency; –30°C, +25°C, and +85°C
22
0.1GHz
21
0.9GHz
SLOPE – mV/dB
UP
20
V
19
2.5
3.0 3.5 4.0 4.5 5.0 5.5 VS – Volts
1.9GHz
2.5GHz
–55
–60
–65
INTERCEPT – dBV
UP
V
–70
–75
0 0.5 1.0
+858C
1.5 2.0 2.5
FREQUENCY – GHz
–308C
+258C
Figure 10. VUP Intercept vs. Frequency: –30°C, +25°C, and +85
°
C
–61
–62
–63
–64
INTERCEPT – dBV
–65
UP
V
–66
–67
2.5
3.0 3.5 4.0 4.5 5.0 5.5 VS – Volts
0.1GHz
2.5GHz
0.9GHz
1.9GHz
Figure 8. VUP Slope vs. Supply Voltage
3500
3000
2500
2000
1500
RESISTANCE – V
1000
500
0
0
X
FREQUENCY (GHz)
0.1
0.9
1.9
2.5
R
0.5 1.0 FREQUENCY – GHz
R
|| - jXV
3030
|| - j748V
760
|| - j106V
301
|| - j80V
90
|| - j141V
X
R
1.5 2.0 2.5
0
–200
–400
–600
–800
–1000
–1200
–1400
REACTANCE – V
Figure 9. Input Impedance
Figure 11. VUP Intercept vs. Supply Voltage
6
5
4
DECREASING
3
2
1
SUPPLY CURRENT – mA
0
–1
0.2
V
ENBL
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 V
ENBL
– Volts
INCREASING V
ENBL
Figure 12. Supply Current vs. ENBL Voltage, VS = 3 V
REV. 0
–5–
AD8314
g
1
2
3
4
ENBL
RFIN
AD8314
RF OUT
TEK
TDS784C
SCOPE
TRIG OUT
PICOSECOND
PULSE LABS
PULSE
GENERATOR
HP8648B
SIGNAL
GENERATOR
PULSE
MODULATION
MODE
10MHz REF OUTPUT
EXT TRIG
NC = NO CONNECT
0.1mF
NC
8
7
6
5
VSET
FLTR
V
DN
VPOS
COMM
V
UP
TEK P6204
FET PROBE
TEK P6204
FET PROBE
3.0V
TRIG
52.3V
OUT
PULSE MODE IN
–3dB
3.0V
RF
SPLITTER
TEK P6204
FET PROBE
–3dB
V
GND
DN
GND
V
UP
V
GND
ENBL
HP8648B
SIGNAL
GENERATOR
–33dBV
52.3V
AVERAGE: 128 SAMPLES
500mV/VERT. DIV.
V
DN
500mV/VERT. DIV.
V
UP
V
ENBL
5V PER VERTICAL DIVISION
Figure 13. ENBL Response Time
10MHz REF OUTPUT EXT TRIG
RF OUT
8
AD8314
VPOS
DN
V
V
UP
COMM
7
6
5
NC
1
2
3
4
RFIN
ENBL
VSET
FLTR
1ms PER HORIZONTAL DIVISION
HP8116A
GENERATOR
PULSE OUT
3.0V
0.1mF
TEK P6204
FET PROBE
TEK P6204
FET PROBE
PULSE
TRIG OUT
TRIG
TEK
TDS784C
SCOPE
AVERAGE: 128 SAMPLES
V
1V/VERT. DIV.
DN
V
500mV/
UP
VERT. DIV.
GND
PULSED RF
0.1GHz, –13dBV
100ns PER HORIZONTAL DIVISION
GND
RF INPUT
200mV PER VERTICAL DIVISION
Figure 16. VUP and VDN Response Time, –40 dBm to 0 dBm
NC = NO CONNECT
Figure 14. Test Setup for ENBL Response Time
80 75 70 65
60 55 50 45 40 35 30 25
AMPLITUDE – dB
20 15 10
5 0
–5
100 1k 10k 100k 1M 10M
10
Figure 15. AC Response from VSET to V_DN
FREQUENCY – Hz
0 –10 –20 –30
–40 –50 –60 –70 –80 –90 –100 –110 –120 –130 –140 –150 –160 –170
rees
PHASE – De
–6–
Figure 17. Test Setup for Pulse Response
10.0
1.0
NOISE SPECTRAL DENSITY – mV/ Hz
0.1 100
–50dBm
–40dBm
–20dBm
1k 10k 100k 1M 10M
–30dBm
FREQUENCY – Hz
RF INPUT –70dBm
Figure 18. VDN Noise Spectral Density
–60dBm
REV. 0
AD8314
– V
DN
V
2.3
2.2
4mA
2.1
2.0
1.9
1.8
1.7
2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5
2.7
0mA
2mA
6mA
VS – Volts
Figure 19. Maximum VDN Voltage vs. VS by Load Current
AVERAGE: 128 SAMPLES
500mV/VERT. DIV.
DN
500mV/VERT. DIV.
UP
1ms PER HORIZONTAL DIVISION
2V PER VERTICAL DIVISION
VDN GND
VUP GND
GND
V
UP
V
V
VPOS AND ENABLE
2.3
– V
DN
V
2.2
2.1
2.0
1.9
SHADING INDICATES
63 SIGMA
1.8
1.7
2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5
2.7 VS – Volts
Figure 22. Maximum VDN Voltage vs. VS with 3 mA Load
AVERAGE: 128 SAMPLES
200mV PER VERTICAL DIVISION
V
GND
GND
DN
VPOS AND ENABLE
2V PER VERTICAL DIVISION
100ns PER HORIZONTAL DIVISION
V
DN
Figure 20. Power-On and -Off Response, Measurement Mode
PULSE OUT
49.9V
TRIG OUT
TRIG
TEK
TDS784C
SCOPE
HP8648B
SIGNAL
GENERATOR
–33dBV
52.3V
10MHz REF OUTPUT
RF OUT
1
2
3
4
NC
RFIN
ENBL
VPOS
V DN
AD8314
VSET
FLTR
NC = NO CONNECT
V
COMM
UP
EXT TRIG
8
7
6
5
HP8116A
PULSE
GENERATOR
AD811
732V
TEK P6204
FET PROBE
TEK P6204
FET PROBE
Figure 21. Test Setup for Power-On and -Off Response
Figure 23. Power-On Response, VDN, Controller Mode with VSET Held Low
TRIG
PULSE OUT
49.9V
OUT
TRIG
TEK
TDS784C
SCOPE
HP8648B
SIGNAL
GENERATOR
RF OUT
52.3V
+0.2
10MHz REF OUTPUT
RFIN
1
2
ENBL
AD8314
3
VSET
FLTR
4
NC
NC = NO CONNECT
VPOS
DN
V
UP
V
COMM
EXT TRIG
8
7
NC
6
5
GENERATOR
AD811
732V
TEK P6204
FET PROBE
HP8112A
PULSE
Figure 24. Test Setup for Power-On Response at V_DN Output, Controller Mode with VSET Pin Held Low
REV. 0
–7–
AD8314
Table I. Typical Specifications at Selected Frequencies at 25C (Mean and Sigma)
1 dB Dynamic Range* – dBV
Slope – mV/dB Intercept – dBV High Point Low Point
Frequency – GHz ␮ ␴ ␮␴␮ ␴ ␮␴
0.1 21.3 0.4 –62.2 0.4 –11.8 0.3 –59 0.5
0.9 20.7 0.4 –63.6 0.4 –13.8 0.3 –61.4 0.4
1.9 19.7 0.4 –66.3 0.4 –19 0.7 –64 0.6
2.5 19.2 0.4 –62.1 0.7 –16.4 1.7 –61 1.3
*Refer to Figure 29.
GENERAL DESCRIPTION
The AD8314 is a logarithmic amplifier (log amp) similar in design to the AD8313; further details about the structure and function may be found in the AD8313 data sheet and other log amps produced by Analog Devices. Figure 25 shows the main features of the AD8314 in block schematic form.
The AD8314 combines two key functions needed for the mea­surement of signal level over a moderately wide dynamic range. First, it provides the amplification needed to respond to small signals, in a chain of four amplifier/limiter cells, each having a small-signal gain of 10 dB and a bandwidth of approximately
3.5 GHz. At the output of each of these amplifier stages is a full-wave rectifier, essentially a square-law detector cell, that converts the RF signal voltages to a fluctuating current having an average value that increases with signal level. A further passive detector stage is added ahead of the first stage. Thus, there are five detectors, each separated by 10 dB, spanning some 50 dB of dynamic range. The overall accuracy at the extremes of this total range, viewed as the deviation from an ideal logarithmic response, that is, the law-conformance error, can be judged by reference to Figure 4, which shows that errors across the central 40 dB are moderate. Other curves show how the conformance to an ideal logarithmic function varies with supply voltage, temperature and frequency.
The output of these detector cells is in the form of a differential current, making their summation a simple matter. It can easily be shown that such summation closely approximates a logarith­mic function. This result is then converted to a voltage, at pin V_UP, through a high-gain stage. In measurement modes, this output is connected back to a voltage-to-current (V–I) stage, in such a manner that V_UP is a logarithmic measure of the RF input voltage, with a slope and intercept controlled by the design. For a fixed termination resistance at the input of the AD8314, a given voltage corresponds to a certain power level.
However, in using this part, it must be understood that log amps do not fundamentally respond to power. It is for this reason that we use dBV (decibels above 1 V rms) rather than the commonly used metric of dBm. While the dBV scaling is fixed, independent of termination impedance, the corresponding power level is not. For example, 224 mV rms is always –13 dBV (with one further condition of an assumed sinusoidal waveform; see the Applications section for more information about the effect of waveform on logarithmic intercept), and it corresponds to a power of 0 dBm
when the net impedance at the input is 50 . When this imped­ance is altered to 200 , the same voltage clearly represents a
power level that is four times smaller (P = V
2
/R), that is, –6 dBm.
Note that dBV may be converted to dBm for the special case of a
50 system by simply adding 13 dB (0 dBV is equivalent to
+13 dBm).
Thus, the external termination added ahead of the AD8314 determines the effective power scaling. This will often take the
form of a simple resistor (52.3 will provide a net 50 input)
but more elaborate matching networks may be used. This im­pedance determines the logarithmic intercept, the input power for which the output would cross the baseline (V_UP = zero) if the function were continuous for all values of input. Since this is never the case for a practical log amp, the intercept refers to the value obtained by the minimum-error straight-line fit to the actual graph of V_UP versus P
(more generally, VIN). Again,
IN
keep in mind that the quoted values assume a sinusoidal (CW) signal. Where there is complex modulation, as in CDMA, the calibration of the power response needs to be adjusted accordingly. Where a true power (waveform-independent) response is needed, the use of an rms-responding detector, such as the AD8361, should be considered.
However, the logarithmic slope, the amount by which the output V_UP changes for each decibel of input change (voltage or power) is, in principle, independent of waveform or termination impedance. In practice, it usually falls off somewhat at higher
RFIN
COMM
(PADDLE)
10dB10dB 10dB
OFFSET COMP'N
AD8314
Figure 25. Block Schematic
–8–
10dB
FLTR
DETDETDETDETDET
BAND-GAP
REFERENCE
V-I
I-V
X2
VPOS ENBL
VSET
V
UP
V DN
REV. 0
AD8314
1
2
3
4
ENBL
RFIN
AD8314
8
7
6
5
VSET
FLTR
V DN
VPOS
COMM
V
UP
0.1mF
OPTIONAL (SEE TEXT)
OPTIONAL (SEE TEXT)
V
S
V
DN
V
UP
C
F
V
S
52.3V
INPUT
frequencies, due to the declining gain of the amplifier stages and other effects in the detector cells. For the AD8314, the slope at low frequencies is nominally 21.3 mV/dB, falling almost linearly with frequency to about 19.2 mV/dB at 2.5 GHz. These values are sensibly independent of temperature (see Figure 7) and almost totally unaffected by the supply voltage from 2.7 V to
5.5 V (Figure 8).
Inverted Output
The second provision is the inclusion of an inverting amplifier to the output, for use in controller applications. Most power amplifiers require a gain-control bias that must decrease from a large positive value toward ground level as the power output is required to decrease. This control voltage, which appears at the pin V_DN, is not only of the opposite polarity to V_UP, but also needs to have an offset added in order to determine its most posi­tive value when the power level (assumed to be monitored through a directional coupler at the output of the PA) is minimal.
The starting value of V_DN is nominally 2.25 V, and it falls on a slope of twice that of V_UP, in other words, –43 mV/dB. Figure 26 shows how this is achieved: the reference voltage that determines the maximum output is derived from the on­chip voltage reference, and is substantially independent of the supply voltage or temperature. However, the full output cannot be attained for supply voltages under 3.3 V; Figure 19 shows this dependency. The relationship between V_UP and V_DN is shown in Figure 27.
CURRENTS FROM
DETECTORS
AD8314
I–V
FLTR
BAND-GAP
REFERENCE
V–I
VSET
+2
= 2.25V – 2.0 3 V_UP
V
DN
1.125V
Figure 26. Output Interfaces
2.5
OUTPUT FOR PA CONTROL
V_DN
OUTPUT FOR MEASUREMENT
–50 –40 –30 –20 –10 0
V_UP
INPUT AMPLITUDE – dBV
REV. 0
2.0
1.5
VOLTS
1.0
0.5
0
–60
Figure 27. Showing V_UP and V_DN Relationship
V_UP
V_DN
APPLICATIONS Basic Connections
Figure 28 shows connections for the basic measurement mode. A supply voltage of 2.7 V to 5.5 V is required. The supply to
the VPOS pin should be decoupled with a low inductance 0.1 µF surface mount ceramic capacitor. A series resistor of about 10
may be added; this resistor will slightly reduce the supply voltage to the AD8314 (maximum current into the VPOS pin is approxi­mately 9 mA when V_DN is delivering 5 mA). Its use should be avoided in applications where the power supply voltage is very low (i.e., 2.7 V). A series inductor will provide similar power supply filtering with minimal drop in supply voltage.
Figure 28. Basic Connections for Operation in Measurement Mode
The ENBL pin is here connected to VPOS. The AD8313 may be disabled by pulling this pin to ground when the chip current
is reduced to about 20 µA from its normal value of 4.5 mA.
The logic threshold is around +V
/2 and the enable function
S
occurs in about 1.5 µs. Note, however, further settling time is
generally needed at low input levels.
The AD8314 has an internal input coupling capacitor. This eliminates the need for external ac-coupling. A broadband input
match is achieved in this example by connecting a 52.3 resis-
tor between RFIN and ground. This resistance combines with
the internal input impedance of approximately 3 k to give an overall broadband input resistance of 50 . Several other
coupling methods are possible; these are described in the Input Coupling section.
The measurement mode is selected by connecting VSET to V_UP, which establishes a feedback path and sets the logarithmic slope to its nominal value. The peak voltage range of the measurement extends from –58 dBV to –13 dBV at 0.9 GHz, and only slightly
less at higher frequencies up to 2.5 GHz. Thus, using the 50
termination, the equivalent power range is –45 dBm to 0 dBm. At a slope of 21.5 mV/dB, this would amount to an output span of 967 mV. Figure 29 shows the transfer function for V_UP at a supply voltage of 3 V, and input frequency of 0.9 GHz.
V_DN, which will generally not be used when the AD8314 is used in the measurement mode, is essentially an inverted version of V_UP. The voltage on V_UP and V_DN are related by the equation.
VDN = 2.25 V – 2 V
UP
While V_DN can deliver up to 6 mA, the load resistance on V_UP
should not be lower than 10 k in order that the full-scale output
of 1 V can be generated with the limited available current of
200 µA max. Figure 29 shows the logarithmic conformance
under the same conditions.
–9–
AD8314
1
2
3
4
ENBL
RFIN
AD8314
8
7
6
5
VSET
FLTR
V DN
VPOS
COMM
V
UP
V
S
VDN
V
S
INPUT
VSET
C
F
0.1mF
52.3V
1.2
1.0
0.8
0.6
– Volts
UP
V
0.4
0.2
VS = 3V RT = 52.3V
61dB DYNAMIC RANGE
63dB DYNAMIC RANGE
0
–70 0
(–47dBm)
INTERCEPT
–60
–50 –40 –30 –20
INPUT AMPLITUDE – dBV
–10
(+3dBm)
3
2
1
0
ERROR – dB
–1
–2
–3
Figure 29. VUP and Log Conformance Error vs. Input Level vs. Input Level at 900 MHz
Transfer Function in Terms of Slope and Intercept
The transfer function of the AD8314 is characterized in terms of its Slope and Intercept. The logarithmic slope is defined as the change in the RSSI output voltage for a 1 dB change at the input. For the AD8314, slope is nominally 21.5 mV/dB. So a 10 dB change at the input results in a change at the output of approxi­mately 215 mV. The plot of Log-Conformance (Figure 29) shows the range over which the device maintains its constant slope. The dynamic range can be defined as the range over which the error
remains within a certain band, usually ±1 dB or ±3 dB. In Figure 29, for example, the ±1 dB dynamic range is approxi-
mately 50 dB (from –13 dBV to –63 dBV).
The intercept is the point at which the extrapolated linear response would intersect the horizontal axis (Figure 29). Using the slope and intercept, the output voltage can be calculated for any input level within the specified input range using the equation:
where V
VUP = V
is the demodulated and filtered RSSI output, V
UP
SLOPE
× (P
– PO)
IN
SLOPE
is the logarithmic slope, expressed in V/dB, PIN is the input sig­nal, expressed in decibels relative to some reference level (either dBm or dBV in this case) and P
is the logarithmic intercept,
O
expressed in decibels relative to the same reference level.
For example, at an input level of –40 dBV (–27 dBm), the output voltage will be
V
= 0.020 V/dB × (–40 dBV – (–63 dBV )) = 0.46 V
OUT
dBV vs. dBm
The most widely used convention in RF systems is to specify power
in dBm, that is, decibels above 1 mW in 50 . Specification of
log amp input levels in terms of power is strictly a concession to popular convention; they do not respond to power (tacitly “power absorbed at the input”), but to the input voltage. The use of dBV, defined as decibels with respect to a 1 V rms sine wave, is more pre­cise, although this is still not unambiguous because waveform is also involved in the response of a log amp, which, for a complex input (such as a CDMA signal), will not follow the rms value exactly. Since most users specify RF signals in terms of power—
more specifically, in dBm/50 —we use both dBV and dBm in
specifying the performance of the AD8314, showing equivalent
dBm levels for the special case of a 50 environment. Values in dBV are converted to dBm re 50 by adding 13.
Filter Capacitor
The video bandwidth of both V_UP and V_DN is approximately
3.5 MHz. In CW applications where the input frequency is much higher than this, no further filtering of the demodulated signal will be required. Where there is a low-frequency modulation of the carrier amplitude, however, the low-pass corner must be reduced by the addition of an external filter capacitor, C Figure 28). The video bandwidth is related to C
Video Bandwidth
=
244 10π .( )
kpFC
××+
by the equation
F
1
(see
F
F
Operating in Controller Mode
Figure 30 shows the basic connections for operation in the con­troller mode and Figure 31 shows a block diagram of a typical controller mode application. The feedback from V_UP to VSET is broken and the desired setpoint voltage is applied to VSET from the controlling source (often this will be a DAC). V
will rail
DN
high (2.2 V on a 3.3 V supply, 1.9 V on a 2.7 V supply) when the applied power is less than the value corresponding to the set­point voltage. When the input power slightly exceeds this value,
would, in the absence of the loop via the power amplifier
V
DN
gain pin, decrease rapidly toward ground. In the closed loop, however, the reduction in V
causes the power amplifier to re-
DN
duce its output. This restores a balance between the actual power level sensed at the input of the AD8314 and the demanded value determined by the setpoint. This assumes that the gain control sense of the variable gain element is positive, that is, an increas­ing voltage from V_DN will tend to increase gain. The output swing and current sourcing capability of V_DN are shown in Figures 19 and 20.
Figure 30. Basic Connections for Operation in Controller Mode
POWER
AMPLIFIER
RF INPUT
DIRECTIONAL COUPLER
C
F
UP
V
FLTR
V DN
GAIN CONTROL VOLTAGE
VSET
DAC
AD8314
52.3V
RFIN
Figure 31. Typical Controller Mode Application
–10–
REV. 0
AD8314
R
SHUNT
52.3V
C
IN
AD8314
50V
50V SOURCE
R
IN
C
C
RFIN
V
BIAS
50V SOURCE
C
IN
AD8314
50V
R
IN
C
C
RFIN
V
BIAS
X2
X1
The relationship between the input level and the setpoint voltage follows from the nominal transfer function of the device (V
UP
vs. Input Amplitude, see Figure 1). For example, a voltage of 1 V on VSET is demanding a power level of 0 dBm at RFIN. The cor­responding power level at the output of the power amplifier will be greater than this amount due to the attenuation through the direc­tional coupler.
When connected in a PA control loop, as shown in Figure 31, the voltage V setting up the required averaging time, by choice of C
is not explicitly used, but is implicated in again
UP
. However,
F
now the effective loop response time is a much more complicated function of the PA’s gain-control characteristics, which are very nonlinear. A complete solution requires specific knowledge of the power amplifier.
The transient response of this control loop is determined by the filter capacitor, C
. When this is large, the loop will be uncon-
F
ditionally stable (by virtue of the “dominant pole” generated by this capacitor), but the response will be sluggish. The minimum value ensuring stability should be used, requiring full attention to the particulars of the power amplifier control function. Because this is invariably nonlinear, the choice must be made for the worst-case condition, which usually corresponds to the smallest output from the PA, where the gain function is steepest. In practice, an improvement in loop dynamics can often be achieved by adding a response zero, formed by a resistor in series with C
.
F
Power-On and Enable Glitch
As already mentioned, the AD8314 can be put into a low power mode by pulling the ENBL pin to ground. This reduces the quies-
cent current from 4.5 mA to 20 µA. Alternatively, the supply can
be turned off completely to eliminate the quiescent current. Figures 13 and 23 show the behavior of the V_DN output under these two conditions (in Figure 23, ENBL is tied to VPOS). The glitch that results in both cases can be reduced by loading the V_DN output.
Input Coupling Options
The internal 5 pF coupling capacitor of the AD8314, along with
the low frequency input impedance of 2 kΩ give a high-pass input
corner frequency of approximately 16 MHz. This sets the mini­mum operating frequency. Figure 32 shows three options for input coupling. A broadband resistive match can be implemented by connecting a shunt resistor to ground at RFIN (Figure 32a).
This 52.3 resistor (other values can also be used to select dif-
ferent overall input impedances) resistor combines with the
input impedance of the AD8314 (3 k储2 pF) to give a broad- band input impedance of 50 . While the input resistance and
capacitance (C
and R
IN
) will vary by approximately ±20% from
IN
device to device, the dominance of the external shunt resistor means that the variation in the overall input impedance will be close to the tolerance of the external resistor.
At frequencies above 2 GHz, the input impedance drops below
250 (see Figure 9), so it is appropriate to use a larger value of
shunt resistor. This value is calculated by plotting the input impedance (resistance and capacitance) on a Smith Chart and choosing the best value of shunt resistor to bring the input imped­ance closest to the center of the chart. At 2.5 GHz, a shunt
resistor of 165 is recommended.
A reactive match can also be implemented as shown in Figure 32b. This is not recommended at low frequencies as device toler­ances will dramatically vary the quality of the match because of
REV. 0
–11–
the large input resistance. For low frequencies, Option a or Option c (see below) are recommended.
In Figure 32b, the matching components are drawn as general reactances. Depending on the frequency, the input impedance at that frequency and the availability of standard value components, either a capacitor or an inductor will be used. As in the previous case, the input impedance at a particular frequency is plotted on a Smith Chart and matching components are chosen (shunt or series L, shunt or series C) to move the impedance to the center of the chart. Table II gives standard component values for some popular frequencies. Matching components for other frequencies can be calculated using the input resistance and reactance data over frequency which is given in Figure 9. Note that the reactance is plotted as though it appears in parallel with the input impedance (which it does because the reactance is prima­rily due to input capacitance).
The impedance matching characteristics of a reactive matching network provide voltage gain ahead of the AD8314; this increases the device sensitivity (see Table II). The voltage gain is calculated using the equation:
R
Voltage Gain
= 20
dB
log
2
10
R
1
where R2 is the input impedance of the AD8314 and R1 is the source impedance to which the AD8314 is being matched. Note that this gain will only be achieved for a perfect match. Component tolerances and the use of standard values will tend to reduce the gain.
a. Broadband Resistive
b. Narrowband Reactive
AD8314
C
C
V
C
BIAS
R
IN
IN
STRIPLINE
R
RFIN
ATTN
c. Series Attenuation
Figure 32. Input Coupling Options
Figure 32c shows a third method for coupling the input signal into the AD8314, applicable in applications where the input signal is larger than the input range of the log amp. A series resistor, connected to the RF source, combines with the input impedance
AD8314
1
2
3
4
ENBL
RFIN
AD8314
8
7
6
5
VSET
FLTR
VPOS
COMM
V
UP
+V
S
2.7V
VSET
0V–1.1V
PF081807B
(HITACHI)
PIN BAND 1 +3dBm
PIN BAND 2 +3dBm
1000pF
0dBm
MAX
+V
S
ATTN
15dB
V DN
C
F
220pF
POUT
BAND 2
+32dBm MAX
POUT BAND 1 +35dBm MAX
4.7mF
TO
ANTENNA
49.9V
7 8 5
1 4 3
26
LDC15D190A0007A
BAND
SELECT
0V/2V
3.5V
V
CTL
V
APC
0.1mF
52.3V
of the AD8314 to resistively divide the input signal being applied to the input. This has the advantage of very little power being “tapped off” in RF power transmission applications.
Table II. Recommended Components for X1 and X2 in Figure 32b
Frequency Voltage Gain (GHz) X1 X2 (dB)
0.1 Short 52.3
0.9 33 nH 39 nH 11.8
1.9 10 nH 15 nH 7.8
2.5 1.5 pF 3.9 nH 2.55
Increasing the Logarithmic Slope in Measurement Mode
The nominal logarithmic slope of 21.5 mV/dB (see Figure 7 for the variation of slope with frequency) can be increased to an arbitrarily high value by attenuating the signal between V_UP and VSET as shown in Figure 33. The ratio R1/R2 is set using the equation
R1/R2 = (New Slope/Original Slope) – 1
In the example shown, two 5 k resistors combine to change the
slope at 1900 MHz from 20 mV/dB to 40 mV/dB. The slope can be increased to higher levels. This will, however, reduce the usable dynamic range of the device.
40mV/dB @ 1900MHz
AD8314
V_UP
VSET
R1 5kV
R2 5kV
Table III. Shift in AD8314 Output for Signals with Differing Crest Factors
Correction Factor (Add to Measured
Signal Type Input Level)
Sine Wave 0 dB Square Wave –3.01 dB GSM Channel (All Time Slots On) 0.55 dB CDMA Channel (Forward Link, 3.55 dB
9 Channels On) CDMA Channel (Reverse Link) 0.5 dB PDC Channel (All Time Slots On) 0.58 dB
Mobile Handset Power Control Examples
Figure 34 shows a complete power amplifier control circuit for a dual mode handset. This circuit is applicable to any dual mode handset using TDMA or CDMA technologies. The PF08107B (Hitachi) is driven by a nominal power level of +3 dBm. Some of the output power from the PA is coupled off using an LDC15D190A0007A (Murata) directional coupler. This has a coupling factor of approximately 19 dB for its lower
frequency band (897.5 ± 17.5 MHz) and 14 dB for its upper band (1747.5 ± 37.5 MHz) and an insertion loss of 0.38 dB and 0.45 dB
respectively. Because the PF08107B transmits a maximum power level of +35 dBm, additional attenuation of 15 dB is required before the coupled signal is applied to the AD8314.
Figure 33. Increasing the Output Slope
Effect of Waveform Type on Intercept
Although specified for input levels in dBm (dB relative to 1 mW), the AD8314 fundamentally responds to voltage and not to power. A direct consequence of this characteristic is that input signals of equal rms power but differing crest factors will produce different results at the log amp’s output.
The effect of differing signal waveforms is to shift the effective value of the intercept upwards or downwards. Graphically, this looks like a vertical shift in the log amp’s transfer function. The logarithmic slope, however, is not affected. For example, consider the case of the AD8314 being alternately fed by an unmodulated sine wave and by a single CDMA channel of the same rms power. The AD8314’s output voltage will differ by the equivalent of
3.55 dB (70 mV) over the complete dynamic range of the device (the output for a CDMA input being lower).
Table III shows the correction factors that should be applied to measure the rms signal strength of a various signal types. A sine wave input is used as a reference. To measure the rms power of a square wave, for example, the mV equivalent of the dB value given in the table (20 mV/dB times 3.01 dB) should be subtracted from the output voltage of the AD8314.
Figure 34. A Dual Mode Power Amplifier Control Circuit
–12–
REV. 0
The setpoint voltage, in the range 0 V to 1.1 V, is applied to the
ENBL
RFIN
AD8314
VSET
FLTR
VPOS
COMM
V
UP
V
S
2.7V
V
SET
0V–1.1V
RF INPUT
0dBm
MAX
V
S
ATTN
15dB
V
DN
C
F
220pF
+35dBm
MAX
47mF
TO
ANTENNA
BGY241
+15dBm
2.2mF
680pF
P
IN
0dBm
DC09-73
6 3
4 5
12
3.5V
0.1mF
52.3V
VSET – Volts
–30
0
POUT – dBm
0.2 0.4 0.6 0.8 1.0
–20
–10
0
10
20
30
40
–40
–50
VSET pin of the AD8314. This will typically be supplied by a Digital-to-Analog Converter (DAC). This voltage is compared to the input level to the AD8314. Any imbalance is between VSET and the RF input level is corrected by V_DN, which drives the
(gain control) of the power amplifier. V_DN reaches a
V
APC
maximum value of approximately 1.9 V on a 2.7 V supply (this will be higher for higher supply voltages) while delivering approxi­mately 3 mA to the V
A filter capacitor (C choice of C
will depend to a large degree on the gain control
F
input.
APC
) must be used to stabilize the loop. The
F
dynamics of the power amplifier, something that is frequently poorly characterized, so some trial and error may be necessary. In this example, a 220 pF capacitor gives the loop sufficient speed to follow the GSM and DCS1800 time slot ramping profiles, while still having a stable, critically-damped response.
Figure 35 shows the relationship between the setpoint voltage, V
and output power, at 0.9 GHz. The overall gain control
SET
function is linear in dB for a dynamic range of over 40 dB.
Figure 36 shows a similar circuit for a single band handset power amplifier. The BGY241 (Phillips) is driven by a nominal power level of 0 dBm. A 20 dB directional coupler, DC09-73 (Alpha) is used to couple the signal in this case. Figure 37 shows the rela­tionship between the control voltage and the output power at
0.9 GHz.
In both of these examples, noise on the V_DN pin can be reduced by placing a simple RC low-pass filter between V
and the gain
DN
control pin of the power amplifier. However, the value of the resistor should be kept low to minimize the voltage drop across it due to the dc current flowing into the gain control input.
AD8314
Figure 36. A Single Mode Power Amplifier Control Circuit
40
30
20
10
VSET – Volts
0
POUT – dBm
–10
–20
–30
Figure 35. POUT vs. VSET at 0.9 GHz for Dual Mode Handset Power Amplifier Application
0.2 0.4 0.6 0.8 1.0 1.2
0
REV. 0
Figure 37. POUT vs. VSET at 0.9 GHz for Single Mode Handset
–13–
AD8314
EVALUATION BOARD
Figure 38 shows the schematic of the AD8314 evaluation board. The layout and silkscreen of the component side are shown in Figures 39 and 40. The board is powered by a single supply in the range, 2.7 V to 5.5 V. The power supply is decoupled
R2
52.3V
R1
INPUT
VSET
LK1
R8
(OPEN)
0V
0V
VPOS
SW1
R7
C4 (OPEN)
1
2
3
4
RFIN
ENBL
VSET
FLTR
AD8314
VPOS
V DN
V
COMM
UP
Figure 38. Evaluation Board Schematic
by a single 0.1 µF capacitor. Additional decoupling, in the form
of a series resistor or inductor in R9, can also be added. Table IV details the various configuration options of the evaluation board.
C1
0.1mF R9
8
R3 0V
7
6
5
0V
R4 (OPEN)C2(OPEN)
R5
0V
R6 (OPEN)
C3 (OPEN)
V
POS
V
V UP
DN
Figure 39. Layout of Component Side Figure 40. Silkscreen of Component Side
–14–
REV. 0
AD8314
Table IV. Evaluation Boards Configuration Options
Component Function Default Condition
TP1, TP2 Supply and Ground Vector Pins Not Applicable SW1 Device Enable: When in position A, the ENBL SW1 = A
pin is connected to +V operating mode. In Position B, the ENBL pin is grounded, putting the device in power-down mode.
R1, R2 Input Interface: The 52.3 resistor in position R2 = 52.3 (Size 0603)
R2 combines with the AD8314’s internal input R1 = 0 (Size 0402)
impedance to give a broadband input impedance
of around 50 . A reactive match can be imple-
mented by replacing R2 with an inductor and
R1 (0 ) with a capacitor. Note that the AD8314’s
RF input is internally ac-coupled.
R3, R4, C2, R5, R6, C3 Output Interface: R4, C2, R6, and C3 can be R4 = C2 = R6 = C3 = Open (Size 0603)
used to check the response of V_UP and V_DN R3 = R5 = 0 (Size 0603)
to capacitive and resistive loading. R3/R4 and R5/R6 can be used to reduce the slope of V_UP and V_DN.
C1, R9 Power Supply Decoupling: The nominal supply C1 = 0.1 µF (Size 0603)
decoupling consists of a 0.1 µF capacitor (C1). A R9 = 0 (Size 0603)
series inductor or small resistor can be placed in R9 for additional decoupling.
C4 Filter Capacitor: The response time of V_UP C4 = Open (Size 0603)
and V_DN can be modified by placing a capacitor between FLTR (pin 4) and V_UP.
R7, R8 Slope Adjust: By installing resistors in R7 and R8, R7 = 0 (Size 0603)
the nominal slope of 20 mV/dB can be increased. R8 = Open (Size 0603) See Slope Adjust discussion for more details.
LK1 Measurement/Controller Mode: LK1 shorts LK1 = Installed
V_UP to VSET, placing the AD8314 in measurement mode. Removing LK1 places the AD8314 in controller mode.
and the AD8314 is in
S
REV. 0
–15–
AD8314
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead micro_SOIC
(RM-8)
0.122 (3.10)
0.114 (2.90)
0.122 (3.10)
0.114 (2.90)
0.006 (0.15)
0.002 (0.05) SEATING
PLANE
85
PIN 1
0.0256 (0.65) BSC
0.120 (3.05)
0.112 (2.84)
0.018 (0.46)
0.008 (0.20)
0.199 (5.05)
0.187 (4.75)
41
0.043 (1.09)
0.037 (0.94)
0.011 (0.28)
0.003 (0.08)
0.120 (3.05)
0.112 (2.84)
338 278
C2726–4.5–10/99
0.028 (0.71)
0.016 (0.41)
–16–
PRINTED IN U.S.A.
REV. 0
Loading...