Datasheet

AD8338 Data Sheet
Rev. A | Page 16 of 20
EXPLANATION OF THE GAIN FUNCTION
The signal chain of the AD8338 can be broken down into
three stages. The first stage is a differential voltage-to-current
converter comprised by the input resistors, R
P
and R
N
, of the
VGA. These input resistors may either be the internal 500 Ω
resistors coupled to Pin INPR and Pin INMR, or external
resistors coupled to Pin INPD and Pin INMD. The trans-
resistance of the voltage-to-current converter is R
P
+ R
N
such that the current flowing in the resistors is given by:
NP
INMxINPx
IN
R R
VV
I
+
−
=
(5)
The current in the input resistors, I
IN
, is fed to the second
stage of the AD8338, the VGA core. The VGA core is a fully
differential variable gain current amplifier with a gain range
of 80 dB. In the noninverting gain slope setting (Pin MODE
connected to Pin VBAT), the current gain of the VGA core
spans from −26 dB (V
GAIN
= 0.1 V) to +54 dB (V
GAIN
= 1.1 V). In
numerical gain magnitude, the gain of the VGA core is given by
mV25020/)3480(
,
1002.010
GAIN
GAIN
V
IN
V
IN
VGAOUT
III ××≈×=
−
(6)
The differential output current of the VGA core is fed to the
third stage of the AD8338, a fully differential current-to-voltage
converter comprised by the output amplifiers and their corre-
sponding feedback resistors, R
FBK
. The overall transimpedance
of the current to voltage converter is 2R
FBK
, such that the
differential output voltage of the stage is given by:
FBK
VGAOUTDIFFOUT
RIV ××= 2
,,
(7)
Therefore, the overall voltage gain of the AD8338 is:
( )
34
2
log2080 −








+
×
×××=
NP
FBK
GAIN
RR
R
VdBG
(8)
Alternatively, the gain equation can be expressed as a numerical
gain magnitude,
mV250
10
2
02.0
GAIN
V
NP
FBK
N
RR
R
G ×
+
×
×=
(9)
Equation 8 and Equation 9 show that the gain range of the
AD8338 can be shifted by using external input resistors, R
P
and
R
N
. For example, driving the INPD and INMD pins with an R
P
and R
N
of 50 Ω shifts the gain range of the AD8338 up by 20 dB,
from 20 dB to 100 dB (see Figure 43). Similarly, driving the
INPD and INMD pins with an R
P
and R
N
of 5 kΩ, shifts the
gain range down by 20 dB, from −20 dB to +60 dB.
As shown in Figure 43, when using external resistors to drive
the INPD and INMD pins, short the INPR and INMR pins to
one another to prevent stability issues.
Effects of Using External Resistors
When the gain range is shifted through the use of external
resistors, several trade-offs must be considered. External
resistors connected to Pin INPD and Pin INMD load the
current inputs of the VGA core changing the dynamic behavior
of the block and the −3 dB bandwidth of the AD8338. The
−3 dB bandwidth of the AD8338 with external resistors is
Ω
×








+Ω
×Ω
×=
500
1
500
500
MHz18
EXT
EXT
CL
R
R
f
(10)
For example, with 50 Ω external resistors, the input-referred
noise at maximum gain decreases to approximately 1 nV/√Hz
and the gain range shifts up by 20 dB. However, the −3 dB
bandwidth is reduced from 18 MHz to approximately 1.8 MHz.