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| 型号： | ADA4939-2YCP-EBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 10/24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 18 of 24
Table 11. Output Noise Voltage Density Calculations for Matched Feedback Networks
Input Noise Contribution
Input Noise Term
Input Noise
Voltage Density
Output
Multiplication Factor
Differential Output Noise
Voltage Density Term
Differential Input
vnIN
GN
vnO1 = GN(vnIN)
Inverting Input
inIN
inIN × (RF2)
1
vnO2 = (inIN)(RF2)
Noninverting Input
inIN
inIN × (RF1)
1
vnO3 = (inIN)(RF1)
VOCM Input
vnCM
0
vnO4 = 0
Gain Resistor RG1
vnRG1
(4kTRG1)1/2
RF1/RG1
vnO5 = (RF1/RG1)(4kTRG1)1/2
Gain Resistor RG2
vnRG2
(4kTRG2)1/2
RF2/RG2
vnO6 = (RF2/RG2)(4kTRG2)1/2
Feedback Resistor RF1
vnRF1
(4kTRF1)1/2
1
vnO7 = (4kTRF1)1/2
Feedback Resistor RF2
vnRF2
(4kTRF2)1/2
1
vnO8 = (4kTRF2)1/2
Table 12. Differential Input, DC-Coupled
Nominal Gain (dB)
RF (Ω)
RG (Ω)
RIN, dm (Ω)
Differential Output Noise Density (nV/√Hz)
6
402
200
400
9.7
10
402
127
254
12.4
14
402
80.6
161
16.6
Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, RS = 50 Ω
Nominal Gain (dB)
RF (Ω)
RG1 (Ω)
RT (Ω)
RIN, cm (Ω)
RG2 (Ω)1
Differential Output Noise Density (nV/√Hz)
6
402
200
60.4
301
228
9.1
10
402
127
66.5
205
155
11.1
14
402
80.6
76.8
138
111
13.5
1 RG2 = RG1 + (RS||RT).
Similar to the case of a conventional op amp, the output noise
voltage densities can be estimated by multiplying the input-
referred terms at +IN and IN by the appropriate output factor,
where:
(
)
2
1
N
β
G
+
=
2
is the circuit noise gain.
G1
F1
G1
1
R
β
+
=
and
G2
F2
G2
2
R
β
+
=
are the feedback factors.
When the feedback factors are matched, RF1/RG1 = RF2/RG2, β1 =
β2 = β, and the noise gain becomes
G
F
N
R
β
G
+
=
1
Note that the output noise from VOCM goes to zero in this case.
The total differential output noise density, vnOD, is the root-sum-
square of the individual output noise terms.
∑
=
8
1
i
2
nOi
nOD
v
associated resistor values, input impedance, and output noise
density for both balanced and unbalanced input configurations.
IMPACT OF MISMATCHES IN THE FEEDBACK
NETWORKS
As previously mentioned, even if the external feedback networks
(RF/RG) are mismatched, the internal common-mode feedback
loop still forces the outputs to remain balanced. The amplitudes
of the signals at each output remain equal and 180° out of phase.
The input-to-output differential mode gain varies proportionately
to the feedback mismatch, but the output balance is unaffected.
The gain from the VOCM pin to VO, dm is equal to
2(β1 β2)/(β1 + β2)
When β1 = β2, this term goes to zero and there is no differential
output voltage due to the voltage on the VOCM input (including
noise). The extreme case occurs when one loop is open and the
other has 100% feedback; in this case, the gain from VOCM input
to VO,dm is either +2 or 2, depending on which loop is closed. The
feedback loops are nominally matched to within 1% in most
applications, and the output noise and offsets due to the VOCM
input are negligible. If the loops are intentionally mismatched by a
large amount, it is necessary to include the gain term from VOCM
to VO, dm and account for the extra noise. For example, if β1 = 0.5
and β2 = 0.25, the gain from VOCM to VO, dm is 0.67. If the VOCM pin
is set to 2.5 V, a differential offset voltage is present at the output of
(2.5 V)(0.67) = 1.67 V. The differential output noise contribution is
(7.5 nV/√Hz)(0.67) = 5 nV/√Hz. Both of these results are
undesirable in most applications; therefore, it is best to use
nominally matched feedback factors.
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