Datasheet

Data Sheet AD8335
Rev. B | Page 17 of 28
PREAMP
Although the preamp signal path is fully differential, the design is
optimized for single-ended input drive and signal source resistance
matching. Thus, the negative input to the differential preamplifier
PMDx pins must be ac-grounded to provide a balanced differential
signal at the PrA outputs. Detailed information regarding the
preamplifier architecture is found in the LNA section of the
AD8331/AD8332 data sheet.
The preamplifier consists of a fixed gain amplifier with differential
outputs. With the negative output available and a fixed gain of
8 (18.06 dB), an active input termination is synthesized by
connecting a feedback resistor between the negative output
and the positive input, Pin PIPx. This technique is well known
and results in the input resistance shown in Equation 2.
)2/1( A
R
R
FB
IN
+
=
(2)
where A/2 is the single-ended gain, or the gain from the PIPx
inputs to the PONx outputs. Since the amplifier has a gain of ×8
from its input to its differential output, it is important to note
that the gain A/2 is the gain from Pin PIPx to Pin PONx, which
is 6 dB lower, or 12.04 dB (×4). The input resistance is reduced
by an internal bias resistor of 14.7 kΩ in parallel with the source
resistance connected to Pin PIPx, with Pin PMDx ac-grounded.
Equation 3 can be used to calculate the needed R
FB
for a desired
R
IN
, and is used for higher values of R
IN
.
k7.14||
)41( +
=
FB
IN
R
R
(3)
For example, to set R
IN
= 200 Ω, the value of R
FB
is 1.013 kΩ. If the
simplified Equation 2 is used to calculate R
IN
, the value is 197 Ω,
resulting in a less than 0.1 dB gain error. Factors such as a widely
varying source resistance might influence the absolute gain
accuracy more significantly. At higher frequencies, the input
capacitance of the PrA needs to be considered. The user must
determine the level of matching accuracy and adjust R
FB
accordingly.
The bandwidths (BW) of the preamplifier and VGA are
approximately 110 MHz each, resulting in a cascaded BW of
approximately 80 MHz. Ultimately the BW of the PrA limits the
accuracy of the synthesized R
IN
. For R
IN
= R
S
up to approximately
200 Ω, the best match is between 100 kHz and 10 MHz, where
the lower frequency limit is determined by the size of the ac
coupling capacitors, and the upper limit is determined by the
preamplifier BW. Furthermore, the input capacitance and R
S
limits the BW at higher frequencies.
INPUT IMPEDANCE (Ω)
FREQUENCY (Hz)
04976-102
10
100
1k
100k 1M 10M 50M
R
IN
= 500Ω, R
FB
= 2.5kΩ
R
SH
=
∞
, C
SH
= 0pF
R
IN
= 200Ω, R
FB
= 1kΩ
R
SH
= 50Ω, C
SH
= 22pF
R
IN
= 100Ω, R
FB
= 499Ω
R
IN
= 50Ω, R
FB
= 249Ω
R
SH
=
∞
, C
SH
= 0pF
R
SH
= 50Ω, C
SH
= 22pF
Figure 55. R
IN
vs. Frequency for Various Values of R
FB
;
Effects of R
SH
and C
SH
are also shown.
Figure 55 shows R
IN
vs. frequency for various values of R
FB
. Note
that at the lowest value, 50 Ω, R
IN
peaks at frequencies greater than
10 MHz. This is due to the BW roll-off of the PrA as mentioned
earlier. The R
SH
and C
SH
network shown in Figure 58 reduces
this peaking.
However, as can be seen for larger R
IN
values, parasitic capacitance
starts rolling off the signal BW before the PrA can produce
peaking and the R
SH
/C
SH
network further degrades the match.
Therefore, R
SH
and C
SH
should not be used for values of R
IN
greater than 50 Ω.
Noise
The total input referred noise (IRN) is approximately 1.3 nV/√Hz.
Allowing for a gain of ×8 in the preamp, the VGA noise is
0.46 nV/√Hz referred to the PrA input. The preamp noise is
1.2 nV/√Hz. It is important to note that these noise values include
all amplifier noise sources, including the VGA and the preamplifier
gain resistors. Frequently, manufacturer noise specifications
exclude gain setting resistors, and the voltage noise spectral density
of an op amp might be presented as 1 nV/√Hz. Including the
gain resistors results in a much higher noise specification.