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| 型号： | ADA4939-2YCP-EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 9/24页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADA4939-2YCP |
| 标准包装： | 1 |
| 每 IC 通道数： | 2 - 双 |
| 放大器类型： | 差分 |
| 板类型： | 裸（未填充） |
| 已供物品： | 板 |
| 已用 IC / 零件： | 24-LFCSP 封装 |
ADA4939-1/ADA4939-2
Rev. 0 | Page 17 of 24
THEORY OF OPERATION
The ADA4939 differs from conventional op amps in that it has
two outputs whose voltages move in opposite directions and an
additional input, VOCM. Like an op amp, it relies on high open-
loop gain and negative feedback to force these outputs to the
desired voltages. The ADA4939 behaves much like a standard
voltage feedback op amp and facilitates single-ended-to-differential
conversions, common-mode level shifting, and amplifications of
differential signals. Like an op amp, the ADA4939 has high input
impedance and low output impedance. Because it uses voltage
feedback, the ADA4939 manifests a nominally constant gain-
bandwidth product.
Two feedback loops are employed to control the differential and
common-mode output voltages. The differential feedback, set
with external resistors, controls only the differential output voltage.
The common-mode feedback controls only the common-mode
output voltage. This architecture makes it easy to set the output
common-mode level to any arbitrary value within the specified
limits. 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 internal common-mode feedback loop produces outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components. This results in
differential outputs that are very close to the ideal of being
identical in amplitude and are exactly 180° apart in phase.
ANALYZING AN APPLICATION CIRCUIT
The ADA4939 uses high open-loop gain and negative feedback
to force its differential and common-mode output voltages in
such a way as to minimize the differential and common-mode
error voltages. The differential error voltage is defined as the
voltage between the differential inputs labeled +IN and IN
(see Figure 42). For most purposes, this voltage can be assumed
to be zero. Similarly, the difference between the actual output
common-mode voltage and the voltage applied to VOCM can also
be assumed to be zero. Starting from these two assumptions,
any application circuit can be analyzed.
SETTING THE CLOSED-LOOP GAIN
The differential-mode gain of the circuit in Figure 42 can be
determined by
G
F
dm
IN
dm
OUT
R
V
=
,
This presumes that the input resistors (RG) and feedback resistors
(RF) on each side are equal.
STABLE FOR GAINS ≥2
The ADA4939 frequency response exhibits excessive peaking
for differential gains <2; therefore, the part should be operated
with differential gains ≥2.
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4939 can be estimated
using the noise model in Figure 43. The input-referred noise
voltage density, vnIN, is modeled as a differential input, and the
noise currents, inIN and inIN+, appear between each input and
ground. The output voltage due to vnIN is obtained by multiplying
vnIN by the noise gain, GN (defined in the GN equation that
follows). The noise currents are uncorrelated with the same
mean-square value, and each produces an output voltage that is
equal to the noise current multiplied by the associated feedback
resistance. The noise voltage density at the VOCM pin is vnCM.
When the feedback networks have the same feedback factor, as
in most cases, the output noise due to vnCM is common-mode.
Each of the four resistors contributes (4kTRxx)1/2. The noise
from the feedback resistors appears directly at the output, and
the noise from the gain resistors appears at the output multiplied
multiplication factors, and the output-referred noise density terms.
ADA4939
+
RF2
VnOD
VnCM
VOCM
VnIN
RF1
RG2
RG1
VnRF1
VnRF2
VnRG1
VnRG2
inIN+
inIN–
07
42
9-
0
50
Figure 43. Noise Model
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