AD604
APPLICATIONS INFORMATION
The basic circuit in Figure 43 shows the connections for one
channel of the AD604. The signal is applied at Pin 5. RGN is
normally 0, in which case the preamplifier is set to a gain of 5
(14 dB). When FBK1 is left open, the preamplifier is set to a
gain of 10 (20 dB) and the gain range shifts up by 6 dB. The ac
coupling capacitors before −DSX1 and +DSX1 should be selected
according to the required lower cutoff frequency. In this example,
the 0.1 μF capacitors, together with the 175 Ω seen looking into
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
bandwidth of the channel, which is 40 MHz. Note that the signal
can be simply inverted by connecting the output of the preamplifier
to −DSX1 instead of +DSX1; this is due to the fully differential
input of the DSX.
0.1µF
1
VREF requires a voltage of 1.25 V to 2.5 V, with between 40 dB/V
and 20 dB/V gain scaling, 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 VGNx is grounded, the channel powers down
and disables its output.
COM1 is the main signal ground for the preamplifier and needs
to be connected with as short a connection as possible to the input
ground. Because the internal feedback resistors of the preamplifier
are very small for noise reasons (8 Ω and 32 Ω nominally), it is
of utmost importance to keep the resistance in this connection
to a minimum. Furthermore, excessive inductance in this
connection can lead to oscillations.
Because of the ultralow noise and wide bandwidth of the
AD604, large dynamic currents flow to and from the power
supply. To ensure the stability of the part, extreme attention to
supply decoupling is required. A large storage capacitor in
parallel with a smaller high frequency capacitor connected at
the supply pins, together with a ferrite bead coming from the
supply, should be used to ensure high frequency stability.
To provide for additional flexibility, COM1 can be used to
disable the preamplifier. When COM1 is connected to VP, the
preamplifier is off, yet the DSX portion can be used independently.
This may be of value when cascading the two DSX stages in the
AD604. In this case, the first DSX output signal with respect to
noise is large and using the second preamplifier at this point
would waste power (see Figure 44).
–DSX1
+DSX1
PAO1
FBK1
PAI1
VGN1
24
VREF
23
OUT1
22
GND1
21
VPOS
20
VNEG
19
VNEG
18
VPOS
17
GND2
16
OUT2
15
VGN
0.1µF
+2.5V
OUT
R
L
500Ω
+5V
–5V
0.1µF
RGN
V
IN
2
3
4
5
6
7
8
9
AD604
COM1
COM2
PAI2
FBK2
10
PAO2
11
+DSX2
12
–DSX2
VGN2
13
0.1µF
Figure 43. Basic Connections for a Single Channel
In Figure 43, the output is ac-coupled for optimum
performance. For dc coupling, as shown in Figure 52, the capacitor
can be eliminated if VOCM is biased at the same 3.3 V common-
mode voltage as the analog-to-digital converter,
00540-043
VOCM
14
Rev. D | Page 18 of 32
AD [ ANALOG DEVICES ]
