AD605
APPLICATIONS
The basic circuit in Figure 38 shows the connections for one
channel 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
Pin −IN1 and Pin +IN1 should be selected according to the
required lower cutoff frequency. In this example, the 0.1 μF
capacitors, together with the 175 Ω of each of the DSX input
pins, provide 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
0.1µF
1
2
VGN
C1
0.1µF
1
2
VGN1
–IN1
+IN1
GND1
GND2
+IN2
–IN2
VGN2
VREF
16
2.500V
R1
VIN
C2
0.1µF
C3
0.1µF
3
4
5
6
7
AD605
OUT1
15
FBK1
14
VPOS
13
VPOS
12
FBK2
11
OUT2
10
VOCM
9
R2
5V
C5
0.1µF
OUT
C6
0.1µF
00541-040
C4
0.1µF
8
VGN1
–IN1
+IN1
GND1
GND2
+IN2
–IN2
VGN2
VREF
16
2.500V
OUT
0.1µF
5V
Figure 39. Doubling the Gain Range with Two Amplifiers
V
IN
0.1µF
3
4
5
6
7
8
AD605
OUT1
15
FBK1
14
VPOS
13
VPOS
12
FBK2
11
OUT2
10
0.1µF
Figure 38. Basic Connections for a Single Channel
As shown in Figure 38, the output is ac-coupled for optimum
performance. In the case of connecting to the 10-bit, 40 MSPS
ADC,
ac coupling can be eliminated as long as
Pin VOCM is biased by the same 3.3 V common-mode
voltage as the
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
powers down and disables its output.
00541-039
VOCM
9
Two other easy combinations are possible to provide a gain
range of −14 dB to +82.8 dB: make R1 a short and R2 an open,
or make R1 an open and R2 a short. The bandwidth for both of
these cases is dominated by the channel that is set to the higher
gain and is about 8 MHz. From a noise standpoint, the second
choice is the best because by increasing the gain of the first
amplifier, the second amplifier’s noise has less of an impact on
the total output noise. One further observation regarding noise
is that by increasing the gain, the output noise increases
proportionally; therefore, there is
no
increase in signal-to-noise
ratio. It actually stays fixed.
It should be noted that by selecting the appropriate values of R1
and R2, any gain range between −28 dB to +68.8 dB and 0 dB to
+96.8 dB can be achieved with the circuit in Figure 39. When
using any value other than shorts and opens for R1 and R2, the
final value of the gain range depends on the external resistors
matching the on-chip resistors. Since the internal resistors can
vary by as much as ±20%, the actual values for a particular gain
have to be determined empirically. Note that the two channels
within one part match quite well; therefore, R1 tracks R2 in
C3 is not required because the common-mode voltage at
Pin OUT1 should be identical to the one at Pin +IN2 and
Pin −IN2. However, since only 1 mV of offset at the output of
the first DSX introduces an offset of 53 mV when the second
DSX is set to the maximum gain of the lowest gain range
(34.4 dB), and 263 mV when set to the maximum gain of the
highest gain range (48.4 dB), it is important to include ac
coupling to get the maximum dynamic range at the output of
the cascaded amplifiers. C5 is necessary if the output signal
needs to be referenced to any common-mode level other than
half of the supply as is provided by Pin OUT2.
CONNECTING TWO AMPLIFIERS TO DOUBLE THE
GAIN RANGE
series to provide a total gain range of 96.8 dB. When R1 and R2
are shorts, the gain range is 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, that is, Pin FBK1 and
Pin FBK2 are left unconnected, then the gain range shifts up by
28 dB to 0 dB to 96.8 dB. As previously noted, the bandwidth of
each individual channel is reduced by a factor of 5 to about
8 MHz because the gain increased by 14 dB. In addition, there is
still the √2 reduction because of the series connection of the
two channels that results in a final bandwidth of the higher gain
version of about 6 MHz.
Rev. D | Page 16 of 20
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