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| 型号： | AD8132ARM-EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 17/33页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD8132ARM |
| 标准包装： | 1 |
| 每 IC 通道数： | 1 - 单 |
| 放大器类型： | 差分 |
| 板类型： | 裸（未填充） |
| 已供物品： | 板 |
| 已用 IC / 零件： | 8-MSOP 封装 |
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AD8132
Rev. I | Page 23 of 32
OTHER β2 = 1 CIRCUITS
The preceding simple configuration with β2 = 1 and its gain of 2
is the highest gain circuit that can be made under this condition.
Because β1 was equal to 0, only higher β1 values are possible.
The circuits with higher values of β1 have gains lower than 2.
However, circuits with β1 equal to 1 are not practical because
they have no effective input and result in a gain of 0.
To increase β1 from 0, it is necessary to add two resistors in a feed-
back network. A generalized circuit that has β1 with a value higher
than 0 is shown in Figure 69. A couple of different convenient
gains that can be created are a gain of 1, when β1 is equal to 1/3,
and a gain of 0.5, when β1 equals 0.6.
With β2 equal to 1 in these circuits, VOCM serves as the refer-
ence voltage that measures the input voltage and the individual
output voltages. In general, when VOCM is varied in circuits with
unmatched feedback networks, a differential output signal is
generated that is proportional to the applied VOCM voltage.
VARYING β2
Though the β2 = 1 circuit sets β2 to 1, another class of simple
circuits can be made that sets β2 equal to 0. This means that
there is no feedback from +OUT to IN. This class of circuits
is very similar to a conventional inverting op amp. However,
the AD8132 circuits have an additional output and common-
mode input that can be analyzed separately (see Figure 71).
With IN connected to ground, +IN becomes a virtual ground
in the sense that the term is used for conventional op amps. Both
inputs must maintain the same voltage for equilibrium operation;
therefore, if one is set to ground, the other is driven to ground.
The input impedance can also be seen to be equal to RG, just as
in a conventional op amp.
In this case, however, the positive input and negative output are
used for the feedback network. Because a conventional op amp
does not have a negative output, only its inverting input can be
used for the feedback network. The AD8132 is symmetrical,
therefore, the feedback network on either side can be used to
produce the same results.
Because +IN is a summing junction, by an analogy to conven-
tional op amps, the gain from VIN to OUT is RF/RG. This holds
true regardless of the voltage on VOCM, and because +OUT
moves the same amount in the opposite direction from OUT,
the overall gain is 2(RF/RG).
VOCM still governs VOUT,cm; therefore, +OUT must be the only
output that moves when VOCM is varied. Because VOUT,cm is the
average of the two outputs, +OUT must move twice as far, and in
the same direction as VOCM, to create the proper VOUT,cm. Therefore,
the gain from VOCM to +OUT must be 2.
With β2 equal to 0 in these circuits, the gain can theoretically be
set to any value from close to 0 to infinity, just as it can with a
conventional op amp in the inverting mode. However, practical
real-world limitations and parasitics limit the range of acceptable
gain to more modest values.
β1 = 0
There is yet another class of circuits where there is no feedback
from OUT to +IN. This is the case where β1 = 0. The differential
amplifier without a resistor described in the Differential Amplifier
section meets this condition, but it was presented only with the
condition that β2 = 1. Recall that this circuit had a gain equal to 2.
If β2 decreases in this circuit from unity, a smaller part of +VOUT
is fed back to IN and the gain increases (see Figure 68). This
circuit is very similar to a noninverting op amp configuration,
except for the presence of the additional complementary output.
Therefore, the overall gain is twice that of a noninverting op
amp or 2 × (1 + RF2/RG2) or 2 × (1/β2).
Once again, varying VOCM does not affect both outputs in the
same way; therefore, in addition to varying VOUT, cm with unity
gain, there is also an effect on VOUT, dm by changing VOCM.
ESTIMATING THE OUTPUT NOISE VOLTAGE
Similar to the case of a conventional op amp, the differential
output errors (noise and offset voltages) can be estimated by
multiplying the input-referred terms, at +IN and IN, by the
circuit noise gain. The noise gain is defined as
+
=
G
F
N
R
G
1
To compute the total output-referred noise for the circuit of
Figure 64, consideration must be given to the contribution of
voltage densities at various closed-loop gains.
Table 11. Recommended Resistor Values and Noise
Performance for Specific Gains
Gain
RG (Ω)
RF (Ω)
Bandwidth
3 dB (MHz)
Output
Noise
AD8132
Only
(nV/√Hz)
Output
Noise
AD8132
+ RG, RF
(nV/√Hz)
1
499
360
16
17
2
499
1.0 k
160
24.1
26.1
5
499
2.49 k
65
48.4
53.3
10
499
4.99 k
20
88.9
98.6
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