AD AD8368 800 mhz, linear-in-db vga with agc detector Datasheet

800 MHz, Linear-in-dB
VGA with AGC Detector
AD8368
FEATURES
FUNCTIONAL BLOCK DIAGRAM
Analog variable gain range: −12 db to +22 dB
Linear-in-dB scaling: 37.5 dB/V
3 dB bandwidth: 800 MHz @ VGAIN = 0.5 V
Integrated rms detector
P1 dB: 16 dBm @ 140 MHz
Output IP3: 33 dBm @ 140 MHz
Noise figure at maximum gain: 9.5 dB @ 140 MHz
Input and output impedances: 50 Ω
Single-supply voltages from 4.5 V to 5.5 V
RoHS-compliant, 24-lead LFCSP
VPSO
MODE
10
21
VPSO VPSI VPSI VPSI VPSI VPSI
9
11
12
22
ICOM 16
GAIN INTERPOLATOR
FIXED-GAIN
AMPLIFIER
OCOM 7
OUTPUT
BUFFER
gm STAGES
–4dB
–36dB
REF
INPT 19
ICOM 18
50Ω
DECL
ATTENUATOR LADDER
–
X2
ENBL
8
OUTP
3
HPFL
4
DECL
14
DECL
15
DECL
+
ICOM 20
APPLICATIONS
Complete IF AGC amplifiers
Gain trimming and leveling
Cellular base station
Point-to-point radio links
RF instrumentation
24
2
5
DETO
DETI
05907-001
0dB –2dB
ICOM 17
13
AD8368
OCOM 6
GAIN 1
23
Figure 1.
GENERAL DESCRIPTION
The AD8368 is a variable gain amplifier with analog linearin-dB gain control that can be used from low frequencies to
800 MHz. Its excellent gain range, conformance, and flatness
are attributed to Analog Devices’ X-AMP® architecture, an
innovative technique for implementing high performance
variable gain control.
The gain range of −12 dB to +22 dB is scaled accurately to
37.5 dB/V with excellent conformance error. The AD8368 has
a 3 dB bandwidth of 800 MHz that is nominally independent
of gain setting. At 140 MHz, the OIP3 is 33 dBm at maximum gain. The output noise floor is –143 dBm/Hz, which
corresponds to a 9.5 dB noise figure at maximum gain. The
single-ended input and output impedances are nominally 50 Ω.
The gain of the AD8368 can be configured to be an increasing
or decreasing function of the gain control voltage depending on
whether the MODE pin is pulled to the positive supply or to
ground, respectively. When MODE is pulled high, the AD8368
operates as a typical VGA with increasing gain. By connecting
MODE to ground and using the on-board rms detector, the
AD8368 can be configured as a complete AGC system with
RSSI. The output power is accurately leveled to the internal
default setpoint of 63 mV rms (−11 dBm referenced to 50 Ω),
independent of the waveform crest factor. Because the
uncommitted detector input is available at DETI, the AGC loop
can level the signal at the AD8368 output or at any other point
in the signal chain over a maximum input power range of
34 dB. Furthermore, the setpoint level can be raised by dividing
down the output signal before applying it to the detector.
The AD8368 operates from a supply voltage of 4.5 V to 5.5 V
and consumes 60 mA of current. It can be fully powered down
to <3 mA by grounding the ENBL pin. The AD8368 is
fabricated using Analog Devices’ proprietary SiGe SOI
complementary bipolar IC process. It is available in a 24-lead
CSP and operates over the industrial temperature range of
−40°C to +85°C. Application boards are available upon request.
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 that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2006 Analog Devices, Inc. All rights reserved.
AD8368
TABLE OF CONTENTS
Features .............................................................................................. 1
Fixed-Gain Stage and Output Buffer ....................................... 12
Applications....................................................................................... 1
Output Offset Correction.......................................................... 12
Functional Block Diagram .............................................................. 1
Input and Output Impedances ................................................. 12
General Description ......................................................................... 1
Gain Control Interface............................................................... 13
Revision History ............................................................................... 2
Applications..................................................................................... 14
Specifications..................................................................................... 3
VGA Operation .......................................................................... 14
Absolute Maximum Ratings............................................................ 5
AGC Operation .......................................................................... 14
ESD Caution.................................................................................. 5
Evaluation Board ............................................................................ 17
Pin Configuration and Function Descriptions............................. 6
Outline Dimensions ....................................................................... 18
Typical Performance Characteristics ............................................. 7
Ordering Guide .......................................................................... 18
Circuit Description......................................................................... 12
Input Attenuator and Interpolator ........................................... 12
REVISION HISTORY
4/06—Revision 0: Initial Version
Rev. 0 | Page 2 of 20
AD8368
SPECIFICATIONS
VS = 5 V, T = 25°C, system impedance Z0 = 50 Ω, VMODE = 5 V, RF input = 140 MHz, unless otherwise noted.
Table 1.
Parameter
OVERALL FUNCTION
Frequency Range
Maximum Input
Maximum Output 1
AC Input Impedance
AC Output Impedance
GAIN CONTROL INTERFACE (GAIN)
Gain Span
Gain Scaling
Gain Accuracy
Maximum Gain
Minimum Gain
VGAIN Range
Gain Step Response
GAIN Input Impedance
GAIN Input Bias Current
f = 70 MHz
Noise Figure
Output IP3
Output P1dB1
f = 140 MHz
Noise Figure
Output IP3
Output P1dB1
f = 240 MHz
Noise Figure
Output IP3
Output P1dB1
f = 380 MHz
Noise Figure
Output IP3
Output P1dB1
Min
Typ
LF
Max
Unit
Conditions
800
MHz
Vp
Vp
Ω
Ω
3 dB bandwidth
To avoid input overload
To avoid clipping
From INPT to ICOM
From OUTP to OCOM
3
2
50
50
34
37.5
−38
±0.4
22
−12
0
1
100
10
−2
dB
dB/V
dB/V
dB
dB
dB
V
ns
kΩ
μA
VMODE = 5 V, 50 mV ≤ VGAIN ≤ 950 mV
VMODE = 0 V, 50 mV ≤ VGAIN ≤ 950 mV
100 mV ≤ VGAIN ≤ 900 mV
VGAIN = 1 V
VGAIN = 0 V
For 6 dB gain step
From GAIN to ICOM
9.5
34
16
dB
dBm
dBm
Maximum gain
f1 = 70 MHz, f2 = 71 MHz, VGAIN = 1 V, 0 dBm per output tone
VGAIN = 0 V, VMODE = 0 V
9.5
33
16
dB
dBm
dBm
Maximum gain
f1 = 140 MHz, f2 = 141 MHz, VGAIN = 1 V, 0 dBm per output tone
VGAIN = 0 V, VMODE = 0 V
9.7
33
15
dB
dBm
dBm
Maximum gain
f1 = 240 MHz, f2 = 241 MHz, VGAIN = 1 V, 0 dBm per output tone
VGAIN = 0 V, VMODE = 0 V
10
29
13
dB
dBm
dBm
Maximum gain
f1 = 380 MHz, f2 = 381 MHz, VGAIN = 1 V, 0 dBm per output tone
VGAIN = 0 V, VMODE = 0 V
1
Operation at compression is not recommended due to adverse distortion components.
Rev. 0 | Page 3 of 20
AD8368
VS = 5 V, T = 25°C, system impedance Z0 = 50 Ω, VMODE = 5 V, RF input = 140 MHz, unless otherwise noted.
Table 2.
Parameter
SQUARE LAW DETECTOR (DETI, DETO)
Output Setpoint
DETI DC Bias Level to ICOM
DETI Impedance
Min
DETO Output Range 1
AGC Step Response
MODE CONTROL INTERFACE (MODE)
Mode Threshold
MODE Input Bias Current
POWER INTERFACE (VPSI, VPSO)
Supply Voltage
Total Supply Current
Disable Current
ENABLE INTERFACE (ENBL)
Enable Threshold
Enable Response Time
0.1
Max
−11
VS/2
710
0.6
VS/2
30
3.5
50
4.5
ENBL Input Bias Current
1
Typ
5
60
2
5.5
Unit
Conditions
dBm
V
Ω
pF
V
μs
OUTP connected to DETI
V
μA
V
mA
mA
2.5
1.5
V
μs
3
μs
150
For −6 dB input power step (CDETO = 1 nF)
μA
Refer to AGC operation in the Applications section.
Rev. 0 | Page 4 of 20
ENBL HIGH
ENBL LOW
Time delay following off to on transition until
output reaches 90% of final value.
Time delay following on to off transition until
supply current is less than 5 mA.
VENBL = 5 V
AD8368
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter
Supply Voltage, VPSO, VPSI
ENBL and MODE Select Voltage
RF Input Level
Internal Power Dissipation
θJA
Maximum Junction Temperature
Operating Temperature Range
Storage Temperature Range
Lead Temperature (Soldering 60 sec)
Rating
5.5 V
5.5 V
20 dBm
440 mW
52°C/W
125°C
−40°C to +85°C
−65°C to +150°C
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 conditions for extended periods may affect
device reliability.
ESD 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 this product 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 | Page 5 of 20
AD8368
ENBL
VPSI
VPSI
MODE
ICOM
INPT
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
24
23
22
21
20
19
GAIN 1
18 ICOM
DETO 2
17 ICOM
HPFL 3
AD8368
16 ICOM
DECL 4
TOP VIEW
(Not to Scale)
15 DECL
DETI 5
14 DECL
8
9
10
11
12
VPSO
VPSO
VPSI
VPSI
05907-002
7
OUTP
13 VPSI
OCOM
OCOM 6
Figure 2. AD8368 24-Lead LFCSP Pin Out
Table 4. Pin Function Descriptions
Pin No.
1
2
3
Name
GAIN
DETO
HPFL
4, 14, 15
DECL
5
6, 7
16, 17, 18, 20
8
9, 10
DETI
OCOM
ICOM
OUTP
VPSO
11, 12, 13,
22, 23
19
21
24
VPSI
INPT
MODE
ENBL
Function
Gain Control.
Detector Output. Provides an output error current for the AGC function.
High-Pass Filter Connection. A capacitor to ground sets the corner frequency of the internal output offset
control loop which controls the minimum usable input frequency.
Decoupling Pin. Nominally ~VS/2. Decoupling capacitance may need to be adjusted for AGC operation (see
the Applications section).
Detector Input. DC level referenced to DECL pin.
Connect OCOM to low impedance ground.
Connect ICOM to low impedance ground.
Signal Output. Must be ac-coupled.
Positive Supply Voltage, 4.5 V to 5.5 V. VPSO and VPSI must be connected together externally and properly
bypassed.
Positive Supply Voltage, 4.5 V to 5.5 V. VPSO and VPSI must be connected together externally and properly
bypassed.
Signal Input. Must be ac-coupled.
Gain Direction Control. HIGH for positive slope. LOW for negative slope.
Apply a positive voltage (2.5V ≤ VENBL ≤ VPSI ) to activate device.
Rev. 0 | Page 6 of 20
AD8368
TYPICAL PERFORMANCE CHARACTERISTICS
VS = 5 V, T = 25°C, system impedance Z0 = 50 Ω, MODE = 5 V, unless otherwise noted.
25
4
25
240MHz
+85°C
10
1
5
0
–1
0
+25°C
0.25V
–40°C
–5
–10
100
–15
1000
0
0.2
0.4
FREQUENCY (MHz)
0.6
VGAIN (V)
Figure 3. Frequency Response vs. VGAIN
Figure 6. Gain and Conformance Error vs. VGAIN (f = 240 MHz)
4
25
25
4
70MHz
380MHz
3
10
1
5
0
–1
+25°C
–40°C
–5
–2
–10
–3
–15
0
0.2
0.4
0.6
–4
1.0
0.8
3
15
GAIN (dB)
GAIN (dB)
+85°C
CONFORMANCE ERROR (dB)
2
15
20
2
+85°C
10
1
5
0
0
–1
+25°C
–40°C
–5
–2
–10
05907-004
20
0
–4
1.0
0.8
–15
CONFORMANCE ERROR (dB)
–20
10
–3
–10
05907-003
0V
–15
–2
05907-006
GAIN (dB)
S21 (dB)
0.5V
0
–5
2
15
0.75V
10
5
3
20
CONFORMANCE ERROR (dB)
1V
15
–3
0
0.2
0.4
VGAIN (V)
0.6
05907-007
20
–4
1.0
0.8
VGAIN (V)
Figure 4. Gain and Conformance Error vs. VGAIN (f = 70 MHz)
Figure 7. Gain and Conformance Error vs. VGAIN (f = 380 MHz)
4
25
0.7
140MHz
20
3
15
2
0.6
5
0
–1
0
+25°C
–40°C
–5
–2
–10
–3
–15
–4
1.0
0
0.2
0.4
0.6
0.8
VGAIN (V)
0.3
0.2
VOUTP
0.1
0
–0.1
–0.2
05907-008
1
VGAIN
0.4
AMPLITUDE (V)
10
05907-005
GAIN (dB)
+85°C
CONFORMANCE ERROR (dB)
0.5
–0.3
–0.4
–0.5
–0.4
–0.3
–0.2
–0.1
0
0.1
0.2
0.3
0.4
TIME (µs)
Figure 8. Gain Step Time Domain Response (6 dB Gain Step)
Figure 5. Gain and Conformance Error vs. VGAIN (f = 140 MHz)
Rev. 0 | Page 7 of 20
0.5
AD8368
20
+85°C
+25°C
–40°C
20
15
10
5
0
70
110
150
190
230
270
310
350
+25°C
16
14
12
10
–40°C
8
6
4
05907-012
OUTPUT 1dB COMPRESSION (dBm)
30
25
+85°C
18
35
05907-009
OUTPUT THIRD-ORDER INTERCEPT (dBm)
40
2
0
70
380
110
150
RF INPUT (MHz)
70MHz
140MHz
350
380
240MHz
380MHz
20
15
10
5
0
0.2
0.4
0.6
0.8
16
14
380MHz
12
240MHz
10
8
6
4
05907-013
OUTPUT 1dB COMPRESSION (dBm)
30
25
140MHz
18
35
05907-010
OUTPUT THIRD-ORDER INTERCEPT (dBm)
310
20
70MHz
2
0
1.0
0
0.2
0.4
VGAIN (V)
0.6
0.8
1.0
VGAIN (V)
Figure 13. Output Compression Point vs. VGAIN (VMODE = 0 V)
Figure 10. OIP3 vs. VGAIN (VMODE = 0 V)
20
0
5.5V
18
–20
–30
–40
380MHz
240MHz
–50
–70
140MHz
0
0.2
0.4
0.6
70MHz
0.8
05907-011
–60
14
5.0V
12
4.5V
10
8
6
4
2
0
70
1.0
110
150
190
230
270
310
350
380
RF INPUT (MHz)
VGAIN (V)
Figure 11. Third-Order IMD vs. VGAIN
(Output Power Is 0 dBm per Tone, VMODE = 0 V)
16
05907-014
OUTPUT 1dB COMPRESSION (dBm)
–10
THIRD-ORDER IMD (dBc)
270
Figure 12. Output Compression Point vs. Frequency at Maximum Gain
(VMODE = 0 V)
40
–80
230
RF INPUT (MHz)
Figure 9. OIP3 vs. Frequency at Maximum Gain (VMODE = 0 V)
0
190
Figure 14. Output Compression Point vs. Supply Voltage at Maximum Gain
(VMODE = 0 V)
Rev. 0 | Page 8 of 20
AD8368
50
45
35
VGAIN = 0.75V
30
25
VGAIN = 0V
20
VGAIN = 1V
15
VGAIN = 0.25V
10
0
10
100
05907-018
05907-015
VGAIN = 0.5V
5
1000
FREQUENCY (MHz)
Figure 18. Input Reflection Coefficient vs. Frequency
Figure 15. Noise Figure vs. Frequency at Maximum Gain (VMODE = 0 V)
50
0
45
–5
OUTPUT RETURN LOSS (dB)
34
30
25
20
15
70MHz
140MHz
240MHz
380MHz
10
5
0
0
0.2
0.4
0.6
0.8
–10
–15
–20
–25
–30
–35
05907-016
NOISE FIGURE (dB)
40
VGAIN = 1V
VGAIN = 0V
–40
10
1.0
100
VGAIN (V)
1000
FREQUENCY (MHz)
Figure 16. Noise Figure vs. VGAIN (VMODE = 0 V)
Figure 19. Output Reflection Coefficient vs. Frequency
0
–5
VGAIN = 0.75V
–15
VGAIN = 1V
VGAIN = 1V
–25
–30
VGAIN = 0V
VGAIN = 0.25V
–40
10
100
VGAIN = 0V
05907-020
–35
05907-017
INPUT RETURN LOSS (dB)
VGAIN = 0.5V
–10
–20
1000
FREQUENCY (MHz)
Figure 17. Input Return Loss vs. Frequency
Figure 20. Output Reflection Coefficient vs. Frequency
Rev. 0 | Page 9 of 20
05907-019
NOISE FIGURE (dB)
40
1.0
0.9
0.8
0.9
0.8
0.8
0.6
0.8
0.4
0.6
0.2
0.5
0
–0.2
+25°C
–0.4
+25°C
0.2
–0.6
0.1
–0.8
–40°C
–35
+85°C
–30
–25
–20
–15
–10
–5
0
5
–1.0
0.6
0.2
0.5
0
–40°C
0.4
0.2
–0.6
0.1
–0.8
0
–40
+25°C
+85°C
–35
–30
–25
0.6
0.7
0.4
0.6
0.2
0.5
0
+25°C
+85°C
–0.2
–40°C
–0.4
–40°C
0.2
–0.6
0.1
–0.8
+85°C
–25
–20
–15
–10
–5
0
5
–1.0
0.8
0.8
0.6
0.7
0.4
0.6
0.2
0.5
0
+25°C
+85°C
–0.2
–40°C
–0.4
–40°C
0.2
–0.6
0.1
0
–40
–0.8
–35
+85°C
–30
–25
–20
–15
–10
–5
0
5
–1.0
SUPPLY CURRENT (mA)
1.0
0.9
CONFORMANCE ERROR (dB)
1.0
+25°C
–1.0
410mV
Figure 25. AGC Time Domain Response (3 dB Power Step, CDETO =1 nF)
05907-023
RSSI (V)
Figure 22. RSSI (VDETO) and Conformance Error vs. Input Power (f = 140 MHz)
0.3
5
VOUTP
CH2 50mV Ω CH3 100mV Ω M20µs 500MS/s A CH1
2.0ns/PT
RF INPUT (dBm)
0.4
0
05907-025
0.8
–30
–5
VRSSI
AMPLITUDE (V)
0.8
–35
–10
Figure 24. RSSI (VDETO) and Conformance Error vs. Input Power (f = 380 MHz)
CONFORMANCE ERROR (dB)
1.0
0.9
0
–40
–15
05907-022
RSSI (V)
1.0
+25°C
–20
RF INPUT (dBm)
Figure 21. RSSI (VDETO) and Conformance Error vs. Input Power (f = 70 MHz)
0.3
–0.4
–40°C
RF INPUT (dBm)
0.4
–0.2
+25°C
0.3
RF INPUT (dBm)
80
8
70
7
60
4.5V
6
50
40
5
5.0V
5.5V
5.5V
4
30
3
5.0V
20
2
10
1
4.5V
0
–40
–20
0
20
40
60
80
0
TEMPERATURE (°C)
Figure 23. RSSI (VDETO) and Conformance Error vs. Input Power (f = 240 MHz)
Rev. 0 | Page 10 of 20
Figure 26. Supply Current and Disable Current vs. Supply Voltage
DISABLE CURRENT (mA)
0
–40
0.4
05907-026
0.4
0.3
–40°C
+85°C
0.7
RSSI (V)
0.7
0.6
+85°C
CONFORMANCE ERROR (dB)
1.0
05907-024
1.0
CONFORMANCE ERROR (dB)
1.0
05907-021
RSSI (V)
AD8368
AD8368
50
VENBL
PERCENTAGE (%)
AMPLITUDE (V)
40
VOUTP
30
20
CH2 500mV Ω CH3 5V Ω M2.0µs 250MS/s
4.0ns/PT
A CH3
0
0.0V
30
20
10
05907-028
PERCENTAGE (%)
40
37.2
37.4
37.6
–14.7
–14.4
–14.1
–13.8
–13.5
Figure 29. Gain Intercept Distribution (140 MHz)
50
37.0
–15.0
INTERCEPT (dB)
Figure 27. ENBL Response Time
0
36.8
05907-029
05907-027
10
37.8
38.0
38.2
SLOPE (dB/V)
Figure 28. Gain Scaling Distribution (140 MHz)
Rev. 0 | Page 11 of 20
AD8368
CIRCUIT DESCRIPTION
The main signal path, shown in Figure 30, consists of a variable
input attenuator followed by a fixed-gain amplifier and output
stage. This architecture allows for a constant OIP3 and output
noise floor as a function of gain setting. As a result, NF and IIP3
increase 1 dB for every 1 dB decrease in gain, resulting in a part
with constant dynamic range over gain setting.
OUTPUT OFFSET CORRECTION
The dc level at the input, INPT, is driven by an internal
reference to VS/2. The reference is made available at the DECL
pin for external decoupling with CDECL. The dc level at the output,
OUTP, is regulated to the same midsupply reference by an offset
correction loop independent of gain setting, temperature, and
process. The low-pass response of this loop creates a high-pass
corner frequency in the signal path transfer function, which can
be set by choosing CDECL and CHPFL.
FIXED-GAIN OUTPUT
AMPLIFIER BUFFER
FROM
INTERPOLATOR
gm STAGES
gm
×1
MODE
GAIN
GAIN INTERPOLATOR
–4dB
HPFL
DECL
CHP
CDECL
VMID
FIXED-GAIN OUTPUT
AMPLIFIER BUFFER
gm STAGES
0dB –2dB
VOUT
VOUT
–36dB
50Ω
DECL
ATTENUATOR LADDER
05907-033
INPT
Figure 30. Simplified Block Diagram
INPUT ATTENUATOR AND INTERPOLATOR
The input attenuator is built from an 18-section resistor ladder
providing 2 dB of attenuation at each successive tap point. The
resistor ladder acts as a linear input attenuator, in addition to
providing an accurate 50 Ω input impedance. The variable
transconductance (gm) stages are used to select the attenuated
signal from the appropriate tap point along the ladder and feed
this signal to the fixed-gain amplifier. To realize a continuous gain
control function from discrete tap points, the gain interpolator
creates a weighted sum of signals appearing on adjacent tap
points by carefully controlling the variable gm stages.
FIXED-GAIN STAGE AND OUTPUT BUFFER
The weighted sum of the different tap points is fed into the
fixed-gain stage which drives the output buffer. Because the
resistive input attenuator is linear and contributes minimal
noise as a passive termination, the dynamic range as a function
of gain is determined primarily by the noise and distortion of
the fixed-gain amplifier. This explains the constant OIP3 and
constant output noise floor with gain setting and the
corresponding dB-for-dB increase in IIP3 and NF with
decreasing gain. The output buffer has 6 dB of gain and
provides a broadband 50 Ω single-ended output impedance.
05907-034
The AD8368 is a single-ended VGA with a bandwidth of
800 MHz and a gain control span of 34 dB ranging from −12 dB
to +22 dB. It incorporates an uncommitted square law detector
that can be used to form a tight AGC loop around the VGA.
Using Analog Devices’ patented X-AMP architecture, the
AD8368 achieves accurate linear-in-dB gain control with
excellent linearity (OIP3) and noise figure (NF). The part also
features 50 Ω input and output impedances for ease of use.
Figure 31. Output Centering Control Loop
The input and output coupling capacitors should be selected to
provide low impedances at the frequencies of interest relative to
50 Ω so as not to affect the high-pass corner. In this case, the
high-pass corner frequency can be set by either CHPFL or CDECL
which form independent poles in the feedback path of the offset
correction loop. The high-pass corner is determined by the
highest of these poles, which are given by:
fHP , HPFL(kHz) =
0.8
(0.005 + CHPFL)
fHP , DECL(kHz) =
5700
(0.005 + CDECL)
where CHPFL and CDECL are in nF.
When using this method to set the high-pass frequency, the
other capacitor should be sized such that its pole is at least
30× lower in frequency. Also note that CDECL represents the total
decoupling capacitance at the DECL pins.
INPUT AND OUTPUT IMPEDANCES
The AD8368 offers single-ended broadband 50 Ω input and
output impedances. The excellent match to 50 Ω is maintained
from part to part, over frequency, and over gain setting. Both
the input and output pins must be externally ac coupled to
prevent disruption of the internal dc levels. Sufficiently large
coupling capacitors should be used so that their impedance is
negligible relative to the 50 Ω presented by the ladder at the
input and by the output buffer at the output.
Rev. 0 | Page 12 of 20
AD8368
GAIN CONTROL INTERFACE
4
25
3
15
2
10
ERROR_L
1
0
5
0
ERROR_H
–1
–2
–5
–3
–10
GAIN_L
The gain functions for MODE pulled high and low are given
respectively by:
–15
where VGAIN is expressed in volts.
0.2
0.4
0.6
0.8
–4
1.0
VGAIN (V)
GainHIGH (dB) = 37.5 × VGAIN − 14
GainLOW (dB) = −38 × VGAIN + 24.8
0
Figure 32. Gain and Conformance Error vs. VGAIN
As shown in Figure 32, the gain function can be either an
increasing or decreasing function of VGAIN, depending on the
MODE pin.
Rev. 0 | Page 13 of 20
05907-035
20
CONFORMANCE ERROR (dB)
GAIN_H
GAIN (dB)
The AD8368 has a linear-in-dB gain control interface that can
be operated in either a gain-up mode or gain-down mode. In
the gain-up mode with the MODE pin pulled high, the gain
increases with increasing gain voltages. In the gain-down mode,
with the MODE pin pulled low, the gain decreases with
increasing gain voltages. In both modes of operation, the gain
control slope is maintained at +37.5 dB/V or −38 dB/V
(depending on mode selection) over temperature, supply, and
process as VGAIN varies from 100 mV to 900 mV. Note that
MODE LOW is required to form an AGC loop with the onboard detector around the VGA.
AD8368
APPLICATIONS
VGA OPERATION
The AD8368 is a general-purpose VGA suitable for use in a wide
variety of applications where accurate, continuous, linear-in-dB
gain control over a broad range of frequencies is important. Its
stability over temperature and supply in comparison to other
variable-gain techniques can be traced back to the X-AMP
architecture. While having an 800 MHz bandwidth, its low
frequency operation can be extended by properly selecting CHPFL
and CDECL.
The typical connections for using the AD8368 in VGA mode
are illustrated in Figure 33. The input (INPT) and output (OUTP)
of the AD8368 should be externally ac coupled to prevent
disrupting the dc levels on the chip. Thus, a sufficiently large
coupling capacitor should be used such that the series impedance
of the capacitor is negligible at the frequencies of interest.
VIN
INPT
VPSI
CDETO
R2
ICOM
X2
DECL
ICOM
REF
–
DECL
+
DECL
OCOM
VPSI
VPSI
AD8368
VPSI
R1
Figure 33. Typical Connections for VGA Mode for Increasing Gain with
Increasing VGAIN (MODE HIGH)
The gain control voltage ranging from 0 V to 1 V is applied to
the GAIN pin. The MODE pin controls whether the gain of the
part is an increasing or decreasing function of the gain voltage.
When the MODE pin is pulled high, the gain increases with
increasing gain voltages. When the MODE pin is pulled low, the
gain decreases with increasing gain voltages. The ENBL pin is
used to enable or disable the part. ENBL is active high; when
ENBL is pulled low, the part is disabled and draws a fraction of
the normal supply current.
INPT
ICOM
DETO
DETI
05907-036
VPOS
VOUT
GAIN
HPFL
VPSI
VPSI
VPSO
OUTP
OCOM
VPSO
AD8368
OCOM
VPOS
VOUT
05907-037
RSSI
DECL
VPSO
+
DETI
ICOM
DECL
MODE
ENBL
ICOM
REF
–
OCOM
X2
VPOS
VPSI
ICOM
VPSI
DETO
VIN
VPSO
ICOM
DECL
The AD8368 can be configured as a standalone AGC amplifier
by using the on-board rms detector, as shown in Figure 34. The
detector output, DETO, is an error current representing the
difference of squares between the root-mean-square (rms) of
the sensed signal and an internal reference of 63 mV rms. This
error current is integrated on CDETO and connected to the GAIN
pin to form the AGC loop.
OUTP
ICOM
MODE
VPSI
VPSI
ENBL
GAIN
HPFL
AGC OPERATION
The 63 mV rms reference corresponds to 178 mV p-p for a sine
wave but the detector accuracy is maintained for more complex
signals such as Gaussian noise, complex envelopes, and multicarrier signals with high peak-to-average ratios.
VPOS
VGAIN
0V TO 1V
The DECL pin provides the internal midsupply dc reference
for the AD8368. It should be well decoupled to ground using
a large capacitor with low ESR. The capacitors connected to
the HPFL pin and DECL pin are used to control the low-pass
corner frequency of the output offset correction loop. The
resulting high-pass corner frequency is inversely proportional
to their values.
Figure 34. AGC Mode of Operation
The AGC mode of operation requires a specific gain direction.
The gain must fall as VDETO increases to restore the needed
balance against the setpoint. Therefore, the MODE pin must
be pulled low. By connecting the signal at OUTP directly to
the detector input (DETI), the output level is driven to the
63 mV rms reference setpoint.
Rev. 0 | Page 14 of 20
AD8368
Figure 36 shows a plot of the RSSI voltage at DETO as input
power is swept.
3.0
2.5
2.0
RSSI (V)
The output setpoint may be increased using an external resistive
divider network between OUTP and DETI, referenced to DECL
as depicted in Figure 34. In this configuration, the rms output
voltage is forced to (1 + R1/R2)63 mV rms by the AGC loop.
For a 0 dBm (224 mV rms referenced to 50 Ω) output setpoint,
this ratio is 3.5. After correcting for the input impedance of
DETI, the choice of R1 = 226 Ω and R2 = 100 Ω yields a
setpoint of roughly 0 dBm. This very accurate leveling function
is shown in Figure 35, where the rms output is held to within
0.2 dB of the 0 dBm setpoint for >30 dB range of input levels.
1.5
1.0
10
5
05907-039
0.5
POWER OUT (dBm)
0
0
–40
–5
–30
–20
–10
0
10
20
POWER IN (dBm)
–10
Figure 36. Monitoring the GAIN/DETO RSSI Voltage vs. Input Power
–15
In some cases, it may be found that, if driven into AGC
overload, the AD8368 requires unusually long times to recover;
that is, the voltage at DETO remains at an abnormally high
value, and the gain is at its lowest value. To avoid this situation,
it is recommended that a clamp be placed on the DETO pin as
shown in Figure 37.
–20
–30
–40
05907-038
–25
–30
–20
–10
0
10
20
POWER IN (dBm)
VPSI
DECL
DECL
VPSI
ICOM
VPSI
VPSO
AD8368
VPSI
+VS
VPSO
OCOM
OCOM
DETI
ENBL
DECL
OUTP
HPFL
VPSI
DETO
CAGC
0.1µF
C20 (pF)
2200
560
150
68
39
GAIN
VAGC
RB
0.5V
A valuable feature of using a square law detector in AGC mode
is that the RSSI voltage is a true reflection of signal power and
may be converted to an absolute power measurement for any
given source impedance. The RSSI in units of dBm referenced
to 50 Ω and based on the voltage available on the DETO pin is
given by
Q1
2N2907
RA
05907-042
C4 (pF)
1000
270
68
33
15
INPT
MODE
Table 5.
IF Frequency (MHz)
70
140
240
380
480
ICOM
Note that to achieve the accurate level of AGC output power,
the DECL capacitor must be adjusted for the corresponding RF
frequency. The DECL capacitor value varies depending on
board parasitics. Table 5 shows the DECL capacitor value based
on the evaluation board parasitics.
ICOM
ICOM
Figure 35. Output Power vs. Input Power in AGC Mode at 140 MHz
Figure 37. External Clamp to Prevent AGC Overload
The resistive divider network, RA and RB, should be designed
such that the base of Q1 is driven to 0.5 V.
RSSI = −11 + 20 log10(1 + R1/R2) + 38 × VDETO − 24.8
Rev. 0 | Page 15 of 20
AD8368
REF –4.9dBm
SR 10kHz
CF 100MHz
16 QAM
MEAS SIGNAL
CONST DIAG
Figure 39 illustrates the measured EVM performance for a
16 QAM modulation at 10 Msymbols/s using CDETO = 1 nF.
10
9
8
7
EVM (%)
The choice of CDETO is a compromise of averaging time constant,
response time, and carrier leakage. If CDETO is selected to be too
small to speed up the response time, the AGC loop could start
tracking and leveling any amplitude envelope and corrupt the
constellation. Figure 38 illustrates a 16 QAM, 100 ksymbols/s
constellation with a degraded error vector magnitude (EVM)
of 5%. By increasing CDETO to 0.01 μF, the EVM is improved
to 1.1%.
1U
6
5
4
3
05907-041
2
1
0
–40
–30
–20
–10
0
10
POWER IN (dBm)
05907-040
Figure 39. Error Vector Magnitude Performance for 16 QAM
10 Msymbols/s
–1U
–1.31289U
262.578mU/
1.31289U
Figure 38. Degraded Error Vector Magnitude Performance for 16 QAM at
100 ksymbols/s(CDETO Too Small)
Rev. 0 | Page 16 of 20
20
AD8368
EVALUATION BOARD
VPOS
C20
1nF
VPSI
VPSI
DECL
DECL
VPSI
OUTP
ENBL
OCOM
R2
10kΩ
GAIN
GAIN
JP4
DET_OUT_TP
R35
OPEN
OUTPUT
C12
1nF
VPOS3
C15
0.1µF
R31
OPEN
C23
10nF
R32
OPEN
C6
1nF
C14
0.1µF
R12
0Ω
COUT
10nF
VPOS2
OCOM
C10
1nF
C11
1nF
VPSO
DETI
OFF
AD8368
DECL
ENABLE
SW2
R11
0Ω
VPSO
MODE
R10
0Ω
ON
VPSI
HPFL
C13
0.1µF
ICOM
DETO
HI
VPOS1
SW1
VPSI
GAIN
LOW
INPT
C1
OPEN
C4
1nF
R30
OPEN
DET_IN
05907-043
R1
10kΩ
ICOM
CIN
10nF
INPUT
ICOM
ICOM
VPOS1 VPOS2 VPOS3
Figure 40.
Table 6. Evaluation Board Configuration Options
Component
R1, R2
R10, R11, R12,
C10, C11, C12,
C13, C14, C15
CIN
COUT
R31, R32
R35
C23
C1, R30
C6
C20, C4
JP4
SW1
SW2
Function
Pull-Down Resistors for Mode and Enable.
Supply Decoupling. Jumpers, power supply decoupling resistors, and filter
capacitors.
RF Input. CIN provides dc block for RF input.
RF Output. CIN provides dc block for RF output.
Feedback Path for AGC Operation. For a default setpoint of 63 mV rms, set R31 = 0 Ω
and remove R32. For other AGC setpoints, the rms voltage is determined from
(1 + n) × 63 mV rms, where n = R31/R32.
Populate with 0 Ω to feed detector output RSSI voltage to DET_OUT_TP.
Sets the corner frequency of output offset control loop high-pass filter.
Used for driving detector externally. Set R30 to 50 Ω for matching. Set C1 to be a
large ac coupling cap.
DETO Capacitor. Needs to be made larger for lower data rates (see the AGC
Operation section).
DECL Capacitor. Needs to be adjusted based on RF frequency in AGC operation
(see the AGC Operation section).
Jumper for AGC Mode of Operation. Provides feedback from the detector output to
the gain pin.
Mode Switch. LOW mode puts the part in gain down mode. HIGH puts the part in
gain up mode. AGC operation requires gain down mode.
Power-Down. The part is disabled when the enable pin is tied to ground.
Rev. 0 | Page 17 of 20
Default Conditions
R1 = R2 = 10 kΩ
R10 = R11 = R12 = 0 Ω
C10 = C11 = C12 = 1 nF
C13 = C14 = C15 = 0.1 μF
CIN = 10 nF
COUT = 10 nF
R31 = R32 = open (VGA mode)
R35 = open
C23 = 10 nF
C1 = not populated
R30 = not populated
C6 = 1 nF
C20 = C4 = 1 nF
JP4 = not populated (VGA mode)
SW1 = JP2
SW2 = JP3
AD8368
OUTLINE DIMENSIONS
0.60 MAX
4.00
BSC SQ
PIN 1
INDICATOR
0.60 MAX
TOP
VIEW
0.50
BSC
3.75
BSC SQ
0.50
0.40
0.30
1.00
0.85
0.80
12° MAX
SEATING
PLANE
0.80 MAX
0.65 TYP
0.30
0.23
0.18
PIN 1
INDICATOR
19
18
24 1
2.65
2.50 SQ
2.35
EXPOSED
PAD
(BOTTOMVIEW)
13
12
7
6
0.23 MIN
2.50 REF
0.05 MAX
0.02 NOM
0.20 REF
COPLANARITY
0.08
COMPLIANT TO JEDEC STANDARDS MO-220-VGGD-8
Figure 41. 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
4 mm × 4 mm Body, Very Thin Quad
(CP-24-4)
Dimensions shown in millimeters
ORDERING GUIDE
Model
AD8368ACPZ-REEL71
AD8368ACPZ-WP1, 2
AD8368-EVALZ1
1
2
Temperature
Range
−40°C to +85°C
−40°C to +85°C
Package Description
24-Lead Lead Frame Chip Scale Package (LFCSP_VQ)
24-Lead Lead Frame Chip Scale Package (LFCSP_VQ)
Evaluation Board
Z = Pb-free part.
WP = Waffle pack.
Rev. 0 | Page 18 of 20
Package
Option
CP-24-4
CP-24-4
Ordering
Quantity
1500
64
AD8368
NOTES
Rev. 0 | Page 19 of 20
AD8368
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05907-0-4/06(0)
Rev. 0 | Page 20 of 20