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AD604 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD604
Beschreibung Dual/ Ultralow Noise Variable Gain Amplifier
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 20 Seiten
AD604 Datasheet, Funktion
a
FEATURES
Ultralow Input Noise at Maximum Gain:
0.80 nV/Hz, 3.0 pA/Hz
Two Independent Linear-in-dB Channels
Absolute Gain Range per Channel Programmable:
0 dB to +48 dB (Preamp Gain = +14 dB), through
+6 dB to +54 dB (Preamp Gain = +20 dB)
؎1.0 dB Gain Accuracy
Bandwidth: 40 MHz (–3 dB)
300 kInput Resistance
Variable Gain Scaling: 20 dB/V through 40 dB/V
Stable Gain with Temperature and Supply Variations
Single-Ended Unipolar Gain Control
Power Shutdown at Lower End of Gain Control
Can Drive A/D Converters Directly
APPLICATIONS
Ultrasound and Sonar Time-Gain Control
High Performance AGC Systems
Signal Measurement
Dual, Ultralow Noise
Variable Gain Amplifier
AD604
FUNCTIONAL BLOCK DIAGRAM
PAO
–DSX
+DSX
VGN
DIFFERENTIAL
ATTENUATOR
R-1.5R
PAI LADDER NETWORK
0 TO –48.4dB
PROGRAMMABLE
ULTRALOW NOISE
PREAMPLIFIER
G = 14–20dB
PRECISION PASSIVE
INPUT ATTENUATOR
GAIN CONTROL
AND SCALING
VREF
AFA
FIXED GAIN
AMPLIFIER
+34.4dB
OUT
VOCM
PRODUCT DESCRIPTION
The AD604 is an ultralow noise, very accurate, dual channel,
linear-in-dB variable gain amplifier (VGA) optimized for time-
based variable gain control in ultrasound applications; however
it will support any application requiring low noise, wide bandwidth
variable gain control. Each channel of the AD604 provides a
300 kinput resistance and unipolar gain control for ease of
use. User determined gain ranges, gain scaling (dB/V) and dc
level shifting of output further optimize application performance.
Each channel of the AD604 utilizes a high performance pre-
amplifier that provides an input referred noise voltage of
0.8 nV/Hz. The very accurate linear-in-dB response of the
AD604 is achieved with the differential input exponential amplifier
(DSX-AMP) architecture. Each of the DSX-AMPs comprise a
variable attenuator of 0 dB to 48.36 dB followed by a high speed
fixed gain amplifier. The attenuator is based on a seven stage
R-1.5R ladder network. The attenuation between tap points
is 6.908 dB and 48.36 dB for the ladder network.
Each independent channel of the AD604 provides a gain range
of 48 dB which can be optimized for the application by program-
ming the preamplifier with a single external resistor in the
preamp feedback path. The linear-in-dB gain response of the
AD604 can be described by the equation: G (dB) = (Gain
Scaling (dB/V ) × VGN (V )) + (Preamp Gain (dB) – 19 dB).
Preamplifier gains between 5 and 10 (+14 dB and +20 dB)
provide overall gain ranges per channel of 0 dB through +48 dB
and +6 dB through +54 dB. The two channels of the AD604
can be cascaded to provide greater levels of gain range by bypass-
ing the 2nd channel’s preamplifier. However, in multiple channel
systems, cascading the AD604 with other devices in the AD60x
VGA family, which do not include a preamplifier may provide
a more efficient solution. The AD604 provides access to the
output of the preamplifier allowing for external filtering be-
tween the preamplifier and the differential attenuator stage.
The gain control interface provides an input resistance of
approximately 2 Mand scale factors from 20 dB/V to
30 dB/V for a VREF input voltage of 2.5 V to 1.67 V respect-
ively. Note that scale factors up to 40 dB/V are achievable
with reduced accuracy for scales above 30 dB/V. The gain scales
linear-in-dB with control voltages of 0.4 V to 2.4 V with the
20 dB/V scale. Below and above this gain control range, the gain
begins to deviate from the ideal linear-in-dB control law. The
gain control region below 0.1 V is not used for gain control. In
fact when the gain control voltage is <50 mV the amplifier
channel is powered down to 1.9 mA.
The AD604 is available in a 24-pin plastic SSOP, SOIC and DIP,
and is guaranteed for operation over the –40°C to +85°C
temperature range.
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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 World Wide Web Site: http://www.analog.com
Fax: 617/326-8703
© Analog Devices, Inc., 1996






AD604 Datasheet, Funktion
AD604–Typical Performance Characteristics (per Channel)
(Unless otherwise noted G (preamp) = +14 dB, VREF = 2.5 V (20 dB/V Scaling), f = 1 MHz, RL = 500 , CL = 5 pF, TA = +25؇C, VSS = ؎5 V)
50
40 VGN = 2.5V
30
20 VGN = 1.5V
VGN = 2.9V
10 VGN = 0.5V
0
VGN = 0.1V
–10
–20
–30 VGN = 0.0V
–40
–50
100k
1M 10M
FREQUENCY – Hz
100M
Figure 10. AC Response
2.55
2.54 VOCM = 2.50V
2.53
–40°C
2.52
2.51
2.50
2.49 +25°C
2.48
2.47
2.46
2.45
0.2
+85°C
0.7 1.2 1.7
VGN – Volts
2.2
2.7
Figure 11. Output Offset vs. VN
210
190
170
+85°C
150
+25°C
130
110
–40°C
90
0.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9
VGN – Volts
Figure 12. Output Referred Noise vs.
VGN
1000
100
10
1
0.1
0.1 0.5 0.9 1.3 1.7 2.1 2.5 2.9
VGN – Volts
Figure 13. Input Referred Noise vs.
VGN
900
VGN = 2.9V
850
800
750
700
650
600
–40 –20
0 20 40 60
TEMPERATURE – °C
80 90
Figure 14. Input Referred Noise vs.
Temperature
770
VGN = 2.9V
765
760
755
750
745
740
100k
1M
FREQUENCY – Hz
10M
Figure 15. Input Referred Noise vs.
Frequency
10
VGN = 2.9V
1
RSOURCE ALONE
0.1
1
10 100
RSOURCE
1k
Figure 16. Input Referred Noise vs.
RSOURCE
16
15
14 VGN = 2.9V
13
12
11
10
9
8
7
6
5
4
3
2
1
1 10 100 1k 10k
RIN
Figure 17. Noise Figure vs. RSOURCE
40
RS = 240
35
30
25
20
15
10
5
0
0 0.4 0.8 1.2 1.6 2.0 2.4 2.8
VGN – Volts
Figure 18. Noise Figure vs. VGN
–6– REV. 0

6 Page









AD604 pdf, datenblatt
AD604
Gain Control Interface
The gain-control interface provides an input resistance of ap-
proximately 2 Mat Pin VGN1 and gain scaling factors from
20 dB/V to 40 dB/V for VREF input voltages of 2.5 V to 1.25 V
respectively. The gain scales linearly-in-dB for the center 40 dB
of gain range, that is for VGN equal to 0.4 V to 2.4 V for the 20
dB/V scale, and 0.2 V to 1.2 V for the 40 dB/V scale. Figure 40
shows the ideal gain curves for a nominal preamplifier gain of
14 dB which are described by the following equations:
G (20 dB/V) = 20 × VGN – 5, VREF = 2.500 V
G (30 dB/V) = 30 × VGN – 5, VREF = 1.666 V
G (40 dB/V) = 40 × VGN – 5, VREF = 1.250 V
(4)
(5)
(6)
50
45
40
35
40dB/V
30dB/V
20dB/V
30
25
LINEAR-IN-dB RANGE
20 OF AD604 WITH
PREAMPLIFIER
15 SET TO 14dB
10
5
0
0.5 1.0 1.5 2.0 2.5 3.0
–5 GAIN CONTROL VOLTAGE – VGN
Figure 40. Ideal Gain Curves vs. VREF.
From these equations you can see that all gain curves intercept
at the same –5 dB point; this intercept will be 6 dB higher
(+1 dB) if the preamplifier gain is set to +20 dB or 14 dB,
lower (–19 dB) if the preamplifier is not used at all. Outside of
the central linear range, the gain starts to deviate from the ideal
control law but still provides another 8.4 dB of range. For a given
gain scaling you can calculate VREF as shown in Equation 7:
V REF
=
2.500 V × 20 dB
Gain Scale
/V
(7)
Usable gain control voltage ranges are 0.1 V to 2.9 V for
20 dB/V scale and 0.1 V to 1.45 V for the 40 dB/V scale. VGN
voltages of less than 0.1 V are not used for gain control since
below 50 mV the channel (preamp and DSX) is powered down.
This can be used to conserve power and at the same time gate-
off the signal. The supply current for a powered-down channel
is 1.9 mA, the response time to power the device on-or-off, is
less than 1 µs.
Active Feedback Amplifier (Fixed Gain Amp)
To achieve single supply operation and a fully differential input
to the DSX, an active-feedback amplifier (AFA) is utilized. The
AFA is basically an op amp with two gm stages; one of the active
stages is used in the feedback path (therefore the name), while
the other is used as a differential input. Note that the differential
input is an open-loop gm stage that requires that it be highly
linear over the expected input signal range. In this design, the
gm stage that senses the voltages on the attenuator is a distrib-
uted one; for example, there are as many gm stages as there are
taps on the ladder network. Only a few of them are on at any
one time, depending on the gain-control voltage.
The AFA makes a differential input structure possible since one
of its inputs (G1) is fully differential; this input is made up of a
distributed gm stage. The second input (G2) is used for feed-
back. The output of G1 will be some function of the voltages
sensed on the attenuator taps which is applied to a high gain
amplifier (A0). Because of negative feedback, the differential
input to the high gain amplifier has to be zero; this in turn
implies that the differential input voltage to G2 times gm2 (the
transconductance of G2) has to be equal to the differential input
voltage to G1 times gm1 (the transconductance of G1). There-
fore the overall gain function of the AFA is:
V OUT = gm1 × R1+ R2
V ATTEN gm2
R2
(8)
where VOUT is the output voltage, VATTEN is the effective voltage
sensed on the attenuator, (R1+R2)/R2 = 42, and gm1/gm2 =
1.25; the overall gain is thus 52.5 (34.4 dB).
The AFA has additional features: (1) inverting the signal by
switching the positive and negative input to the ladder network,
(2) the possibility of using the DSX1 input as a second signal
input, (3) fully differential high impedance inputs when both
preamplifiers are used with one DSX (the other DSX could still
be used alone), and (4) independent control of the DSX common-
mode voltage. Under normal operating conditions it is best to
connect a decoupling capacitor to pin VOCM in which case the
common-mode voltage of the DSX is half the supply voltage;
this allows for maximum signal swing. Nevertheless, the
common-mode voltage can be shifted up or down by directly
applying a voltage to VOCM. It can also be used as another
signal input, the only limitation being the rather low slew-rate
of the VOCM buffer.
If the dc level of the output signal is not critical, another
coupling capacitor is normally used at the output of the DSX;
again this is done for level shifting and to eliminate any dc off-
sets contributed by the DSX (see AC Coupling section).
–12–
REV. 0

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