AD605(RevC) Просмотр технического описания (PDF) - Analog Devices
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AD605 Datasheet PDF : 12 Pages
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AD605
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 transcon-
ductance of G2) has to be equal to the differential input voltage
to G1 times gm1 (the transconductance of G1). Therefore the
overall gain function of the AFA is
VOUT = gm1 × R1 × R2
VATTEN gm2
R2
(7)
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 output signal
by switching the positive and negative input to the ladder network;
(2) the possibility of using the –IN input as a second signal input;
and (3) independent control of the DSX common-mode voltage.
Under normal operating conditions it is best to just 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 offsets contrib-
uted by the DSX (see AC Coupling section).
The gain range of the DSX is programmable by a resistor con-
nected between pins FBK and OUT. The possible ranges are
–14 dB to +34.4 dB when the pins are shorted together, to 0 dB
to +48.4 dB when FBK is left open. Note that for the higher
gain range, the bandwidth of the amplifier is reduced by a factor
of five to about 8 MHz since the gain increased by 14 dB. This
is the case for any constant gain bandwidth product amplifier
which includes the active feedback amplifier.
APPLICATIONS
The basic circuit in Figure 4 shows the connections for one chan-
nel of the AD605 with a gain range of –14 dB to +34.4 dB. The
signal is applied at Pin 3. The ac-coupling capacitors before pins
–IN1 and +IN1 should be selected according to the required lower
cutoff frequency. In this example, the 0.1 µF capacitors to-
gether with the 175 Ω of each of the DSX input pins provides a
–3 dB high-pass corner of about 9.1 kHz. The upper cutoff
frequency is determined by the amplifier and is 40 MHz.
VGN
VIN
0.1F
0.1F
1 VGN1
VREF 16
2 –IN1
OUT1 15
AD605
3 +IN1
FBK1 14
4 GND1
VPOS 13
5 GND2
VPOS 12
6 +IN2
FBK2 11
7 –IN2
OUT2 10
8 VGN2
VOCM 9
0.1F
5V
0.1F
2.500V
OUT
Figure 4. Basic Connections for a Single Channel
As shown here, the output is ac-coupled for optimum perfor-
mance. In the case of connecting to the 10-bit 40 MSPS A/D
converter AD9050, ac coupling can be eliminated as long as
pin VOCM is biased by the same 3.3 V common-mode voltage
as the AD9050.
Pin VREF requires a voltage of 1.25 V to 2.5 V, with gain
scaling between 40 dB/V and 20 dB/V, respectively. Voltage
VGN controls the gain; its nominal operating range is from
0.25 V to 2.65 V for 20 dB/V gain scaling, and 0.125 V to
1.325 V for 40 dB/V scaling. When this pin is taken to ground,
the channel will power down and disable its output.
Connecting Two Amplifiers to Double the Gain Range
Figure 5 shows the two channels of the AD605 connected in
series to provide a total gain range of 96.8 dB. When R1 and R2
are shorts, the gain range will be from –28 dB to +68.8 dB with
a slightly reduced bandwidth of about 30 MHz. The reduction
in bandwidth is due to two identical low-pass circuits being
connected in series; in the case of two identical single-pole low-
pass filters, the bandwidth would be reduced by exactly √2. If
R1 and R2 are replaced by open circuits, i.e., Pins FBK1 and
FBK2 are left unconnected, then the gain range will shift up by
28 dB to 0 dB to +96.8 dB. As noted earlier, the bandwidth of
each individual channel will be reduced by a factor of 5 to
about 8 MHz since the gain increased by 14 dB. In addition,
there is still the √2 reduction because of the series connection of
the two channels which results in a final bandwidth of the higher
gain version of about 6 MHz.
VGN
C1
0.1F
VIN
C2
0.1F
C3
0.1F
C4
0.1F
1 VGN1
VREF 16
2.500V
2 –IN1
OUT1 15
AD605
R1
3 +IN1
FBK1 14
4 GND1
VPOS 13
5V
5 GND2
VPOS 12
6 +IN2
7 –IN2
FBK2 11
R2
OUT2 10
C5
0.1F
OUT
8 VGN2
VOCM 9
C6
0.1F
Figure 5. Doubling the Gain Range with Two Amplifiers
–10–
REV. C
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