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| 型號： | AD8476-EVALZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 9/24頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD8476 |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 每 IC 通道數(shù)： | 1 - 單 |
| 放大器類型： | 差分 |
| 輸出類型： | 差分，滿擺幅 |
| 轉(zhuǎn)換速率： | 10 V/µs |
| -3db帶寬： | 5MHz |
| 電流 - 輸出 / 通道： | 35mA |
| 工作溫度： | -40°C ~ 125°C |
| 電流供應(yīng)（主 IC）： | 330µA |
| 電壓 - 電源，單路/雙路(±)： | 3 V ~ 18 V，±1.5 V ~ 9 V |
| 板類型： | 完全填充 |
| 已供物品： | 板 |
| 已用 IC / 零件： | AD8476 |
Data Sheet
AD8476
Rev. B | Page 17 of 24
THEORY OF OPERATION
OVERVIEW
The AD8476 is a fully differential amplifier, with integrated laser-
trimmed resistors, that provides a precision gain of 1. The
internal differential amplifier of the AD8476 differs from
conventional operational amplifiers in that it has two outputs
whose voltages are equal in magnitude, but move in opposite
directions (180° out of phase).
The AD8476 is designed to greatly simplify single-ended-to-
differential conversion, common-mode level shifting and
precision driving of differential signals into low power,
differential input ADCs. The VOCM input allows the user to
set the output common-mode voltage to match with the input
range of the ADC. Like an operational amplifier, the VOCM
function relies on high open-loop gain and negative feedback to
force the output nodes to the desired voltages.
10k
INN
1
+V
S
2
V
O
CM
3
+
O
UT
4
IN
P
8
–V
S
7
NC
6
–O
UT
5
NOTES
1. NC = NO CONNECT.
DO NOT CONNECT TO THIS PIN.
10195-
058
AD8476
Figure 51. Block Diagram
CIRCUIT INFORMATION
The AD8476 amplifier uses a voltage feedback topology;
therefore, the amplifier exhibits a nominally constant gain
bandwidth product. Like a voltage feedback operational
amplifier, the AD8476 also has high input impedance at its
internal input terminals (the summing nodes of the internal
amplifier) and low output impedance.
The AD8476 employs two feedback loops, one each to control
the differential and common-mode output voltages. The differen-
tial feedback loop, which is fixed with precision laser-trimmed
on-chip resistors, controls the differential output voltage.
Output Common-Mode Voltage (VOCM)
The internal common-mode feedback controls the common-
mode output voltage. This architecture makes it easy for the
user to set the output common-mode level to any arbitrary
value independent of the input voltage. The output common-
mode voltage is forced by the internal common-mode feedback
loop to be equal to the voltage applied to the VOCM input. The
VOCM pin can be left unconnected, and the output common-
mode voltage self-biases to midsupply by the internal feedback
control.
Due to the internal common-mode feedback loop and the fully
differential topology of the amplifier, the AD8476 outputs are
precisely balanced over a wide frequency range. This means that
the amplifier’s differential outputs are very close to the ideal of
being identical in amplitude and exactly 180° out of phase.
DC PRECISION
The dc precision of the AD8476 is highly dependent on the
accuracy of its integrated gain resistors. Using superposition to
analyze the circuit shown in Figure 52, the following equation
shows the relationship between the input and output voltages of
the amplifier:
(
)
(
)
(
)
(
)
N
P
dm
OUT
N
P
cm
OUT
N
P
N
P
dm
IN
N
P
cm
IN
R
V
R
V
R
V
R
V
+
=
+
2
1
2
1
,
where:
RGP
RFP
RP =
,
RGN
RFN
RN =
N
P
dm
IN
V
=
,
)
(
2
1
,
N
P
cm
IN
V
+
=
The differential closed-loop gain of the amplifier is
N
P
N
P
N
P
dm
IN
dm
OUT
R
V
+
=
2
,
and the common rejection of the amplifier is
(
)
N
P
N
P
cm
IN
dm
OUT
R
V
+
=
2
,
RFP
RFN
RGP
RGN
VON
VOP
VOCM
VP
VN
10195-
059
Figure 52. Functional Circuit Diagram of the AD8476 at a Given Gain
The preceding equations show that the gain accuracy and the
common-mode rejection (CMRR) of the AD8476 are deter-
mined primarily by the matching of the feedback networks
(resistor ratios). If the two networks are perfectly matched, that
is, if RP and RN equal RF/RG, then the resistor network does not
generate any CMRR errors and the differential closed loop gain
of the amplifier reduces to
RG
RF
v
dm
IN
dm
OUT
=
,
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