Analog Devices AD603 Datasheet

Low Noise, 90 MHz

a

FEATURES
“Linear in dB” Gain Control
Pin Programmable Gain Ranges

–11 dB to +31 dB with 90 MHz Bandwidth
+9 dB to +51 dB with 9 MHz Bandwidth

Any Intermediate Range, e.g., –1 dB to +41 dB with

30 MHz Bandwidth

Bandwidth Independent of Variable Gain

1.3 nV/√Hz Input Noise Spectral Density

ⴞ0.5 dB Typical Gain Accuracy
MIL-STD-883 Compliant and DESC Versions Available

APPLICATIONS
RF/IF AGC Amplifier
Video Gain Control
A/D Range Extension
Signal Measurement

PRODUCT DESCRIPTION

The AD603 is a low noise, voltage-controlled amplifier for use
in RF and IF AGC systems. It provides accurate, pin selectable
gains of –11 dB to +31 dB with a bandwidth of 90 MHz or
+9 dB to +51 dB with a bandwidth of 9 MHz. Any intermediate
gain range may be arranged using one external resistor. The

input referred noise spectral density is only 1.3 nV/√Hz and power
consumption is 125 mW at the recommended ±5 V supplies.

The decibel gain is “linear in dB,” accurately calibrated, and
stable over temperature and supply. The gain is controlled at a

high impedance (50 MΩ), low bias (200 nA) differential input;

the scaling is 25 mV/dB, requiring a gain-control voltage of only

Variable-Gain Amplifier

AD603*

1 V to span the central 40 dB of the gain range. An over- and
under-range of 1 dB is provided whatever the selected range. The

gain-control response time is less than 1 µs for a 40 dB change.

The differential gain-control interface allows the use of either
differential or single-ended positive or negative control voltages.
Several of these amplifiers may be cascaded and their gain-con­trol gains offset to optimize the system S/N ratio.

The AD603 can drive a load impedance as low as 100 Ω with
low distortion. For a 500 Ω load in shunt with 5 pF, the total
harmonic distortion for a ±1 V sinusoidal output at 10 MHz is
typically –60 dBc. The peak specified output is ±2.5 V mini­mum into a 500 Ω load, or ±1 V into a 100 Ω load.

The AD603 uses a proprietary circuit topology—the X-AMP™.
The X-AMP comprises a variable attenuator of 0 dB to
–42.14 dB followed by a fixed-gain amplifier. Because of the
attenuator, the amplifier never has to cope with large inputs and
can use negative feedback to define its (fixed) gain and dynamic

performance. The attenuator has an input resistance of 100 Ω,
laser trimmed to ±3%, and comprises a seven-stage R-2R ladder

network, resulting in an attenuation between tap points of

6.021 dB. A proprietary interpolation technique provides a
continuous gain-control function which is linear in dB.

The AD603A is specified for operation from –40°C to +85°C

and is available in both 8-lead SOIC (R) and 8-lead ceramic

DIP (Q). The AD603S is specified for operation from –55°C to
+125°C and is available in an 8-lead ceramic DIP (Q). The

AD603 is also available under DESC SMD 5962-94572.

FUNCTIONAL BLOCK DIAGRAM

VPOS
VNEG

GPOS

GNEG

VINP

COMM

*Patented.
X-AMP is a trademark of Analog Devices, Inc.

SCALING

REFERENCE

V

G

GAIN

CONTROL

INTERFACE

0dB –6.02dB –12.04dB–18.06dB –24.08dB –30.1dB –36.12dB –42.14dB

RRRRRRR

2R 2R 2R 2R 2R 2R R

R = 2R LADDER NETWORK

PRECISION PASSIVE
INPUT ATTENUATOR

REV. C

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.

FIXED GAIN

AMPLIFIER

V

OUT

AD603

*NORMAL VALUES

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] World Wide Web Site: http://www.analog.com
Fax: [phone] © Analog Devices, Inc., 2000

6.44kV*

FDBK

694V*

20V*

AD603–SPECIFICATIONS

(@ TA = +25ⴗC, V
Range, RL = 500 ⍀, and CL = 5 pF, unless otherwise noted.)

= ⴞ5 V, –500 mV ≤ VG ≤ +500 mV, GNEG = 0 V, –10 dB to +30 dB Gain

S

Model AD603
Parameter Conditions Min Typ Max Unit

INPUT CHARACTERISTICS

Input Resistance Pins 3 to 4 97 100 103 Ω

Input Capacitance 2pF
Input Noise Spectral Density
Noise Figure f = 10 MHz, Gain = max, R
1 dB Compression Point f = 10 MHz, Gain = max, R

1

Input Short Circuited 1.3 nV/√Hz

= 10 Ω 8.8 dB

S

= 10 Ω –11 dBm

S

Peak Input Voltage ±1.4 ±2V

OUTPUT CHARACTERISTICS

–3 dB Bandwidth V
Slew Rate R
Peak Output

2

= 100 mV rms 90 MHz

OUT

≥ 500 Ω 275 V/µs

L

R

≥ 500 Ω±2.5 ±3.0 V

L

Output Impedance f ≤ 10 MHz 2 Ω

Output Short-Circuit Current 50 mA

Group Delay Change vs. Gain f = 3 MHz; Full Gain Range ±2ns

Group Delay Change vs. Frequency V

= 0 V; f = 1 MHz to 10 MHz ±2ns

G

Differential Gain 0.2 %
Differential Phase 0.2 Degree
Total Harmonic Distortion f = 10 MHz, V
3rd Order Intercept f = 40 MHz, Gain = max, R

= 1 V rms –60 dBc

OUT

= 50 Ω 15 dBm

S

ACCURACY

Gain Accuracy –500 mV ≤ V

T

to T

MIN

Output Offset Voltage

T

MIN

Output Offset Variation vs. V

T

MIN

to T

to T

MAX

MAX

MAX

3

G

VG = 0 V 20 mV

–500 mV ≤ VG ≤ +500 mV 20 mV

≤ +500 mV ±0.5

G

ⴞ

1 dB

±1.5 dB

30 mV

30 mV

GAIN CONTROL INTERFACE

Gain Scaling Factor 39.4 40 40.6 dB/V

to T

T

MIN

GNEG, GPOS Voltage Range

MAX

4

38 42 dB/V

–1.2 +2.0 V
Input Bias Current 200 nA
Input Offset Current 10 nA

Differential Input Resistance Pins 1 to 2 50 MΩ
Response Rate Full 40 dB Gain Change 40 dB/µs

POWER SUPPLY

Specified Operating Range ±4.75 ±6.3 V

Quiescent Current 12.5 17 mA

T

to T

MIN

NOTES

1

Typical open or short-circuited input; noise is lower when system is set to maximum gain and input is short-circuited. This figure includes the effects of both voltage
and current noise sources.

2

Using resistive loads of 500 Ω or greater, or with the addition of a 1 kΩ pull-down resistor when driving lower loads.

3

The dc gain of the main amplifier in the AD603 is ×35.7; thus, an input offset of 100 µV becomes a 3.57 mV output offset.

4

GNEG and GPOS, gain control, voltage range is guaranteed to be within the range of – VS + 4.2 V to +V

Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min
and max specifications are guaranteed, although only those shown in boldface are tested on all production units.

Specifications subject to change without notice.

MAX

– 3.4 V over the full temperature range of –40°C to +85°C.

S

20 mA

–2–

REV. C

AD603

WARNING!

ESD SENSITIVE DEVICE

ABSOLUTE MAXIMUM RATINGS

1

Supply Voltage ±VS . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±7.5 V

Internal Voltage VINP (Pin 3) . . . . . . . . . . . ±2 V Continuous

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±V

GPOS, GNEG (Pins 1, 2) . . . . . . . . . . . . . . . . . . . . . . . ±V

for 10 ms

S

S

Internal Power Dissipation2 . . . . . . . . . . . . . . . . . . . . 400 mW

Operating Temperature Range

AD603A . . . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C

AD603S . . . . . . . . . . . . . . . . . . . . . . . . . . – 55°C to +125°C

Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C

Lead Temperature Range (Soldering 60 sec) . . . . . . . . +300°C

NOTES

1

Stresses above those listed under Absolute Maximum Ratings may cause perma-

nent 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.

2

Thermal Characteristics:

8-Lead SOIC Package: θJA = 155°C/W, θJC = 33°C/W
8-Lead Ceramic Package: θJA = 140°C/W, θJC = 15°C/W

ORDERING GUIDE

PIN FUNCTION DESCRIPTIONS

Pin Mnemonic Description

Pin 1 GPOS Gain-Control Input “HI”

(Positive Voltage Increases Gain)

Pin 2 GNEG Gain-Control Input “LO”

(Negative Voltage Increases Gain)
Pin 3 VINP Amplifier Input
Pin 4 COMM Amplifier Ground
Pin 5 FDBK Connection to Feedback Network
Pin 6 VNEG Negative Supply Input
Pin 7 VOUT Amplifier Output
Pin 8 VPOS Positive Supply Input

CONNECTION DIAGRAMS

8-Lead Plastic SOIC (R) Package

8-Lead Ceramic DIP (Q) Package

GPOS
GNEG

VINP

1

2

AD603

TOP VIEW

3

(Not to Scale)

4

8

7

6

5

VPOS
VOUT
VNEG
FDBKCOMM

Temperature Package Package

Part Number Range Description Option

AD603AR –40°C to +85°C 8-Lead SOIC SO-8
AD603AQ –40°C to +85°C 8-Lead Ceramic DIP Q-8
AD603SQ/883B* –55°C to +125°C 8-Lead Ceramic DIP Q-8

AD603-EB Evaluation Board

AD603ACHIPS –40°C to +85°C Die
AD603AR-REEL –40°C to +85°C 13" Reel SO-8
AD603AR-REEL7 –40°C to +85°C 7" Reel SO-8

*Refer to AD603 Military data sheet. Also available as 5962-9457203MPA.

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 AD603 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. C

–3–

AD603

THEORY OF OPERATION

The AD603 comprises a fixed-gain amplifier, preceded by a
broadband passive attenuator of 0 dB to 42.14 dB, having a
gain-control scaling factor of 40 dB per volt. The fixed gain is

laser-trimmed in two ranges, to either 31.07 dB (×35.8) or
50 dB (×358), or may be set to any range in between using one

external resistor between Pins 5 and 7. Somewhat higher gain
can be obtained by connecting the resistor from Pin 5 to com­mon, but the increase in output offset voltage limits the
maximum gain to about 60 dB. For any given range, the band­width is independent of the voltage-controlled gain. This system
provides an under- and overrange of 1.07 dB in all cases;
for example, the overall gain is –11.07 dB to 31.07 dB in the
maximum-bandwidth mode (Pin 5 and Pin 7 strapped).

This X-AMP structure has many advantages over former methods
of gain-control based on nonlinear elements. Most importantly,
the fixed-gain amplifier can use negative feedback to increase its
accuracy. Since large inputs are first attenuated, the amplifier

input is always small. For example, to deliver a ±1 V output in

the –1 dB/+41 dB mode (that is, using a fixed amplifier gain of

41.07 dB) its input is only 8.84 mV; thus the distortion can be
very low. Equally important, the small-signal gain and phase
response, and thus the pulse response, are essentially indepen­dent of gain.

Figure 1 is a simplified schematic. The input attenuator is a
seven-section R-2R ladder network, using untrimmed resistors

of nominally R = 62.5 Ω, which results in a characteristic resis­tance of 125 Ω ± 20%. A shunt resistor is included at the input

and laser trimmed to establish a more exact input resistance of

100 Ω ± 3%, which ensures accurate operation (gain and HP

corner frequency) when used in conjunction with external resistors
or capacitors.

The nominal maximum signal at input VINP is 1 V rms (±1.4 V
peak) when using the recommended ±5 V supplies, although
operation to ±2 V peak is permissible with some increase in HF

distortion and feedthrough. Pin 4 (SIGNAL COMMON) must

be connected directly to the input ground; significant impedance in
this connection will reduce the gain accuracy.

The signal applied at the input of the ladder network is attenu­ated by 6.02 dB by each section; thus, the attenuation to each of
the taps is progressively 0 dB, 6.02 dB, 12.04 dB, 18.06 dB,

24.08 dB, 30.1 dB, 36.12 dB and 42.14 dB. A unique circuit
technique is employed to interpolate between these tap-points,

indicated by the “slider” in Figure 1, thus providing continuous
attenuation from 0 dB to 42.14 dB. It will help, in understanding
the AD603, to think in terms of a mechanical means for moving
this slider from left to right; in fact, its “position” is controlled
by the voltage between Pins 1 and 2. The details of the gain­control interface are discussed later.

The gain is at all times very exactly determined, and a linear-in­dB relationship is automatically guaranteed by the exponential
nature of the attenuation in the ladder network (the X-AMP
principle). In practice, the gain deviates slightly from the ideal

law, by about ±0.2 dB peak (see, for example, Figure 16).

Noise Performance

An important advantage of the X-AMP is its superior noise per­formance. The nominal resistance seen at inner tap points is

41.7 Ω (one third of 125 Ω), which exhibits a Johnson noise­spectral density (NSD) of 0.83 nV/√Hz (that is, √4kTR) at 27°C,

which is a large fraction of the total input noise. The first stage

of the amplifier contributes a further 1 nV/√Hz, for a total input
noise of 1.3 nV/√Hz. It will be apparent that it is essential to use

a low resistance in the ladder network to achieve the very low
specified noise level. The signal’s source impedance forms a

voltage divider with the AD603’s 100 Ω input resistance. In

some applications, the resulting attenuation may be unaccept­able, requiring the use of an external buffer or preamplifier to
match a high impedance source to the low impedance AD603.

The noise at maximum gain (that is, at the 0 dB tap) depends
on whether the input is short-circuited or open-circuited: when

shorted, the minimum NSD of slightly over 1 nV/√Hz is achieved;
when open, the resistance of 100 Ω looking into the first tap
generates 1.29 nV/√Hz, so the noise increases to a total of

1.63 nV/√Hz. (This last calculation would be important if the
AD603 were preceded by, for example, a 900 Ω resistor to allow

operation from inputs up to 10 V rms.) As the selected tap
moves away from the input, the dependence of the noise on
source impedance quickly diminishes.

Apart from the small variations just discussed, the signal-to­noise (S/N) ratio at the output is essentially independent of the
attenuator setting. For example, on the –11 dB/+31 dB range

the fixed gain of ×35.8 raises the output NSD to 46.5 nV/√Hz.

Thus, for the maximum undistorted output of 1 V rms and a
1 MHz bandwidth, the output S/N ratio would be 86.6 dB, that

is, 20 log (1 V/46.5 µV).

VPOS
VNEG

GPOS

GNEG

VINP

COMM

SCALING

REFERENCE

V

G

GAIN

CONTROL

INTERFACE

0dB –6.02dB –12.04dB–18.06dB –24.08dB –30.1dB –36.12dB –42.14dB

RRRRRRR

2R 2R 2R 2R 2R 2R R

R = 2R LADDER NETWORK

PRECISION PASSIVE
INPUT ATTENUATOR

AD603

Figure 1. Simplified Block Diagram of the AD603

–4–

FIXED GAIN

AMPLIFIER

6.44kV*

694V*

20V*

*NORMAL VALUES

V

OUT

FDBK

REV. C

AD603

The Gain-Control Interface

The attenuation is controlled through a differential, high-

impedance (50 MΩ) input, with a scaling factor which is

laser-trimmed to 40 dB per volt, that is, 25 mV/dB. An internal
bandgap reference ensures stability of the scaling with respect to
supply and temperature variations.

When the differential input voltage V

= 0 V, the attenuator

G

“slider” is centered, providing an attenuation of 21.07 dB. For
the maximum bandwidth range, this results in an overall gain of
10 dB (= –21.07 dB + 31.07 dB). When the control input is

–500 mV, the gain is lowered by 20 dB (= 0.500 V × 40 dB/V),

to –10 dB; when set to +500 mV, the gain is increased by 20 dB, to
30 dB. When this interface is overdriven in either direction, the
gain approaches either –11.07 dB (= –42.14 dB + 31.07 dB) or

31.07 dB (= 0 + 31.07 dB), respectively. The only constraint on
the gain-control voltage is that it be kept within the common-mode
range (–1.2 V to +2.0 V assuming +5 V supplies) of the gain
control interface.

The basic gain of the AD603 can thus be calculated using the
following simple expression:

Gain (dB) = 40 V

where V

is in volts. When Pins 5 and 7 are strapped (see next

G

+ 10 (1)

G

section) the gain becomes

Gain (dB) = 40 V

+ 20 for 0 to +40 dB

G

and

Gain (dB) = 40 V

+ 30 for +10 to +50 dB (2)

G

The high impedance gain-control input ensures minimal loading
when driving many amplifiers in multiple channel or cascaded
applications. The differential capability provides flexibility in
choosing the appropriate signal levels and polarities for various
control schemes.

For example, if the gain is to be controlled by a DAC providing
a positive only ground-referenced output, the “Gain Control
LO” (GNEG) pin should be biased to a fixed offset of +500 mV,
to set the gain to –10 dB when “Gain Control HI” (GPOS) is at
zero, and to 30 dB when at +1.00 V.

It is a simple matter to include a voltage divider to achieve other
scaling factors. When using an 8-bit DAC having an FS output
of +2.55 V (10 mV/bit), a divider ratio of 2 (generating 5 mV/bit)
would result in a gain-setting resolution of 0.2 dB/bit. The use
of such offsets is valuable when two AD603s are cascaded, when
various options exist for optimizing the S/N profile, as will be
shown later.

Programming the Fixed-Gain Amplifier Using Pin Strapping

Access to the feedback network is provided at Pin 5 (FDBK).
The user may program the gain of the AD603’s output amplifier
using this pin, as shown in Figure 2. There are three modes: in
the default mode, FDBK is unconnected, providing the range
+9 dB/+51 dB; when V

and FDBK are shorted, the gain is

OUT

lowered to –11 dB/+31 dB; when an external resistor is placed
between V

and FDBK any intermediate gain can be achieved,

OUT

for example, –1 dB/+41 dB. Figure 3 shows the nominal maxi­mum gain versus external resistor for this mode.

VC1

VPOS

VPOS

GPOS

AD603

GNEG

VINP

COMM

V

OUT

VNEG

FDBK

VNEG

V

OUT

VC2

V

IN

a. –10 dB to +30 dB; 90 MHz Bandwidth

VC1

VPOS

VPOS

GPOS

AD603

VC2

V

IN

GNEG

VINP

COMM

V

OUT

VNEG

FDBK

VNEG

2.15kV

5.6pF

V

OUT

b. 0 dB to +40 dB; 30 MHz Bandwidth

VC1

VPOS

VPOS

GPOS

AD603

VC2

V

IN

GNEG

VINP

COMM

V

OUT

VNEG

FDBK

VNEG

18pF

V

OUT

c. +10 dB to +50 dB; 9 MHz Bandwidth

Figure 2. Pin Strapping to Set Gain

52
50
48
46
44
42
40

DECIBELS

38
36
34
32
30

10 1M

Figure 3. Gain vs. R

–1:VdB (OUT)

VdB (OUT)

–2:VdB (OUT)

100 1k 10k 100k

EXT

R

EXT

, Showing Worst-Case Limits
Assuming Internal Resistors Have a Maximum Tolerance
of 20%

REV. C

–5–