Datasheet
AD8370 Data Sheet
Rev. B | Page 18 of 28
If the amplifier is driven in single-ended mode, the input
impedance varies depending on the value of the resistor used to
terminate the other input as
Rin
SE
= Rin
DIFF
+ R
TERM
where R
TERM
is the termination resistor connected to the other
input.
–100
–90
–80
–70
–60
–50
–40
HARMONIC DISTORTION (dBc)
09632 640 32 64 96 128
GAIN CODE
03692-048
HD2
HD2
LOW GAIN MODE
HIGH GAIN MODE
HD3
HD3
Figure 50. Harmonic Distortion of the Circuit in Figure 47
DC-COUPLED OPERATION
AD8370
INHI
ICOM
VCCI
PWUP
VOCM
VCCO
OCOM
OPHI
OPLO
OCOM
VCCO
LTCH
CLCK
DATA
ICOM
INLO
6 7 82 3 51
11 10 915 1416 13 12
4
SERIAL CONTROL
INTERFACE
1nF
1nF
0.1µF
0.1µF
–2.5V
–2.5V
0V
0V
+2.5V
R
L
SINGLE-
ENDED
GROUND
REFERENCED
SOURCE
R
S
R
T
03692-049
Figure 51. DC Coupling the AD8370. Dual supplies are used to set the input
and output common-mode levels to 0 V.
03692-050
AD8370AD8138
INHI
ICOM
VCCI
PWUP
VOCM
VCCO
OCOM
OPHI
OPLO
OCOM
VCCO
LTCH
CLCK
DATA
ICOM
INLO
6 7 82 3 51
11 10 915 1416 13 12
4
SERIAL CONTROL
INTERFACE
1nF1nF
0.1µF
+5V
499Ω
499Ω
100Ω
499Ω
499Ω
100Ω
V
OCM
V
OCM
V
OCM
+5V
R
L
SINGLE-ENDED GROUND
REFERENCED SOURCE
R
S
R
T
R
T
2
Figure 52. DC Coupling the AD8370. The AD8138 is used as a unity-gain level
shifting amplifier to lift the common-mode level of the source to midsupply.
The AD8370 is also a dc accurate VGA. The common-mode dc
voltage present at the output pins is internally set to midsupply
using what is essentially a buffered resistive divider network
connected between the positive supply rail and the common
(ground) pins. The input pins are at a slightly higher dc potential,
typically 250 mV to 550 mV above the output pins, depending
on gain setting. In a typical single-supply application, it is
necessary to raise the common-mode reference level of the
source and load to roughly midsupply to maintain symmetric
swing and to avoid sinking or sourcing strong bias currents
from the input and output pins. It is possible to use balanced
dual supplies to allow ground referenced source and load, as
shown in Figure 51. By connecting the VOCM pin and unused
input to ground, the input and output common-mode potentials
are forced to virtual ground. This allows direct coupling of
ground referenced source and loads. The initial differential
input offset is typically only a few 100 µV. Over temperature,
the input offset could be as high as a few tens of mVs. If precise
dc accuracy is needed over temperature and time, it may be
necessary to periodically measure the input offset and to apply
the necessary opposing offset to the unused differential input,
canceling the resulting output offset.
To address situations where dual supplies are not convenient, a
second option is presented in Figure 52. The AD8138 differential
amplifier is used to translate the common-mode level of the driving
source to midsupply, which allows dc accurate performance
with a ground-referenced source without the need for dual
supplies. The bandwidth of the solution in Figure 52 is limited
by the gain-bandwidth product of the AD8138. The normalized
frequency response of both implementations is shown in Figure 53.