Data Sheet
AD8330
Rev. F | Page 19 of 32
noise is critical. This objective is complicated by the fact that
the common-mode level varies with the basic gain voltage, VDBS.
Figure 55 shows this relationship for a supply voltage of 5 V, for
temperatures of 40°C, +25°C, and +85°C
. Figure 56 shows the
input noise spectral density (RS = 0) vs. the input common-
mode voltage, for VDBS = 0.5 V, 0.6 V, 0.75 V, and 1.5 V. It is
apparent that there is a broad range over which the noise is
unaffected by this dc level. The input CMRR is excellent (see
VDBS (V)
0
D
C
V
OLTA
GE
A
T
IN
H
I,
IN
LO
(
V
)
2.6
3.2
3.1
3.0
2.9
2.8
2.7
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
T = +25°C
T = +85°C
T = –40°C
03217-
056
Figure 55. Common-Mode Voltage at Input Pins vs. VDBS, for VS = 5 V,
T = 40°C, + 25°C, and + 85°C
COMMON-MODE VOLTAGE AT INHI, INLO (V)
0
26
22
20
18
16
14
12
10
8
4
6
IN
PU
T
R
EF
ER
R
ED
N
O
ISE
(n
V/
√Hz
)
24
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
VDBS = 1.5V
VDBS = 0.75V
VDBS = 0.6V
VDBS = 0.5V
SIMULATION
03217-
057
Figure 56. Input Noise vs. Common-Mode Input Voltage for
VDBS = 0.5 V, 0.6 V, 0.75 V, and 1.5 V
Output Noise and Peak Swing
The output noise of the AD8330 is the input noise multiplied by
the overall gain, including any optional change to the voltage,
VMAG, applied to Pin VMAG. The peak output swing is also
proportional to this voltage, which, at low gains and high values
of VMAG, affects the output noise.
The scaling for VDBS = 0 V is as follows:
VOUT_PK = ±4 VMAG
(8)
VNOISE_OUT = (85 + 70 VMAG) nV/√Hz
(9)
For example, using a reduced value of VMAG = 0.25 V that lowers
all gain values by 6 dB, the peak output swing is ±1 V (differ-
entially) and the output noise spectral density evaluates to
102.5 nV/√Hz. The peak output swing is no different at full
gain, but the noise becomes
VNOISE_OUT = (0.1 + 0.32 VMAG) V/√Hz
(10)
for RS = 0 and VDBS = 1.5 V, assuming an input noise of 5 nV/√Hz.
The output noise for very small values of VMAG (at or below 15 mV)
is not precise, partly because the small input offset associated
with this interface has a large effect on the gain.
Offset Compensation
The AD8330 includes an offset compensation feature that is
operational in the default condition (no connection to Pin OFST).
This loop introduces a high-pass filter function into the signal
path, whose 3 dB corner frequency is at
(
)
HP
INT
HPF
C
R
f
π
=
2
1
(11)
where:
CHP is the external capacitance added from OFST to CNTR.
RINT is an internal resistance of approximately 480 Ω, having a
maximum uncertainty of about ±20%.
This evaluates to
HP
HPF
C
f
= 330 (CHP in μF)
(12)
A small amount of peaking at this corner when using small
capacitor values can be avoided by adding a series resistor.
Useful combinations are CHP = 3 nF, RHP = 180 Ω, f = 100 kHz;
CHP = 33 nF, RHP = 10 Ω, f = 10 kHz; CHP = 0.33 μF, RHP = 0 Ω,
f = 1 kHz; CHP = 3.3 μF, RHP = 0 Ω, f = 100 Hz.
The offset compensation feature can be disabled simply by
grounding the OFST pin. This provides a dc-coupled signal
path, with no other effects on the overall ac response. Input
offsets must be externally nulled in this mode of operation, as
Effects of Loading on Gain and AC Response
The differential output impedance (RO) is 150 Ω, and the fre-
quency response of the output stage is optimized for operation
with a certain load capacitance on each output pin (OPHI and
OPLO) to ground, in combination with a load resistance (RL)
directly across these pins. In the absence of these capacitances,
there is a small amount of peaking at the top extremity of the
ac response. Suitable combinations are: RL = ∞, CL = 12 pF;
RL = 150 Ω, CL = 25 pF; RL = 75 Ω, CL = 40 pF; or RL = 50 Ω,
CL = 50 pF.
The gain calibration is specified for an open-circuited load,
such as the high input resistance of an ADC. When resistively
loaded, all gain values are nominally lowered as follows:
(
)
L
UNLOADED
LOADED
R
G
+
=
Ω
150
(13)