Analog Devices AD604 - Manual (Page 14)

AD604
PREAMPLIFIER
The input capability of the following single-supply DSX (2.5 ± 2 V
for a +5 V supply) limits the maximum input voltage of the
preamplifier to ±400 mV for the 14 dB gain configuration or
±200 mV for the 20 dB gain configuration.
The preamplifier gain can be programmed to 14 dB or 20 dB by
either shorting the FBK1 node to PAO1 (14 dB) or by leaving
the FBK1 node open (20 dB). These two gain settings are very
accurate because they are set by the ratio of the on-chip resistors.
Any intermediate gain can be achieved by connecting the
appropriate resistor value between PAO1 and FBK1 according
to Equation 2 and Equation 3.
{R7\\ R ext )+ R5
G _ V ouT
R6
R6
[R6 X G - (R5 + R6)] X R7
R7 - (R6 X G) + (R5 + R6)
the gain can be in error by as much as 0.33 dB when R
ext
is 30 O,
where it is assumed that R ext is exact.
Figure 38 shows how the preamplifier is set to gains of 14 dB,
17.5 dB, and 20 dB. The gain range of a single channel of the
AD604 is 0 dB to 48 dB when the preamplifier is set to 14 dB
(Figure 38a), 3.5 dB to 51.5 dB for a preamp g^in If 1
(Figure 38b), a^ j! dB\o^4"dBfor the hig|eiT^r|amp ga|n of
20 dB (Figure 38l5y W W •
PAI1 O-
R6
8Q
COM1 O—VW-
r
R5
320
—Wr-
■OPAO1
R7
400
OFBK1
a. PREAMP GAIN = 14dB
PAM O-
R6
80
COM1 o—w-
R5
320
PAO1
—o—
R7
400
R10
400
b.PREAMP GAIN = 17.5dB
PAI1 O-
R6
80
COM1 O—VA-
r
-OPAO1
R7
R5 ? 400
320
-W. 4------------- OFBK1
c. PREAMP GAIN=20dB
Figure 38. Preamplifier Gain Programmability
preamplifier to be 17.7 dB. The -3 dB small signal bandwidth of
one complete channel of the AD604 (preamplifier and DSX) is
40 MHz and is independent of gain.
To achieve optimum specifications, power and ground manage­
ment are critical to the AD604. Large dynamic currents result
because of the low resistances needed for the desired noise
performance. Most of the difficulty is with the very low gain
setting resistors of the preamplifier that allow for a total input
referred noise, including the DSX, as low as 0.8 nV/VHz. The
consequently large dynamic currents have to be carefully
handled to maintain performance even at large signal levels.
(2) 18
17
(3)
ST
16 ■o
z
15
<
O 14
1 ^ V in
12 - ■=
10M
FREQUENCY (Hz)
I dB, and 20 dB
The prea
dynamic currents and a grou
For a preamplifier gain of 14 dB, the -3 dB small signal bandwidth
of the preamplifier is 130 MHz. When the gain is at its maximum
of 20 dB, the bandwidth is reduced by half to 65 MHz. Figure 39
shows the ac response for the three preamp gains shown in
Figure 38. Note that the gain for an R ext of 40 O should be
17.5 dB, but the mismatch between the internal resistors and
the external resistor causes the actual gain for this particular
Rev. E | Page 14 of 32
Figure 39. AC Response for Preampl^^er Ga
ifier uses a dua ^^V suppy H acfcommodate large
d referenced input. The preamplifier
output is also ground referenced and requires a common-mode
level shift into the single-supply DSX. The two external coupling
capacitors (C1 and C2 in Figure 37) connected to the PAO1 and
+DSXx, and -DSXx, nodes and ground, respectively, perform
this function (see the AC Coupling section). In addition, they
eliminate any offset that would otherwise be introduced by the
preamplifier. It should be noted that an offset of 1 mV at the
input of the DSX is amplified by 34.4 dB (x 52.5) when the gain
control voltage is at its maximum; this equates to 52.5 mV at the
output. AC coupling is consequently required to keep the offset
from degrading the output signal range.
The gain-setting preamplifier feedback resistors are small
enough (8 O and 32 O) that even an additional 1 O in the
ground connection at Pin COM1 (the input common-mode
reference) seriously degrades gain accuracy and noise performance.
This node is sensitive, and careful attention is necessary to
minimize the ground impedance. All connections to the COM1
node should be as short as possible.
The preamplifier, including the gain setting resistors, has a
noise performance of 0.71 nV/VHz and 3 pA/VHz. Note that a
significant portion of the total input referred voltage noise is
due to the feedback resistors. The equivalent noise resistance
presented by R5 and R6 in parallel is nominally 6.4 O, which
contributes 0.33 nV/VHz to the total input referred voltage noise.
20
19
R
E X T
100k 1M 100M
FBK1