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ADA4927-1 PDF预览

ADA4927-1

更新时间: 2024-10-28 06:36:15
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亚德诺 - ADI 驱动器
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Ultralow Distortion Current Feedback Differential ADC Driver

ADA4927-1 数据手册

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ADA4927-1/ADA4927-2  
Table 12. Differential Input, DC-Coupled  
Nominal Gain (dB)  
RF (Ω)  
RG (Ω)  
RIN, dm (Ω)  
602  
Differential Output Noise Density (nV/√Hz)  
0
301  
301  
8.0  
20  
26  
442  
604  
44.2  
30.1  
88.4  
60.2  
21.8  
37.9  
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  
328  
77.2  
74.4  
Differential Output Noise Density (nV/√Hz)  
0
20  
26  
309  
511  
806  
301  
39.2  
28  
56.2  
158  
649  
401  
73.2  
54.2  
8.1  
18.6  
29.1  
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,  
The feedback loops are nominally matched to within 1% in  
most applications, and the output noise and offsets due to the  
VꢀCM input are negligible. If the loops are intentionally mismatched  
by a large amount, it is necessary to include the gain term from  
where:  
2
VꢀCM to Vꢀ, dm and account for the extra noise. For example, if  
GN  
=
is the circuit noise gain.  
β1 = 0.5 and β2 = 0.25, the gain from VꢀCM to Vꢀ, dm is 0.67. If the  
VꢀCM 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 (15 nV/√Hz)(0.67) = 10 nV/√Hz. Both of these  
results are undesirable in most applications; therefore, it is best  
to use nominally matched feedback factors.  
(
β1 + β2  
RG1  
)
RG2  
β1 =  
and β2 =  
are the feedback factors.  
RF1 + RG1  
RF2 + RG2  
When the feedback factors are matched, RF1/RG1 = RF2/RG2,  
β1 = β2 = β, and the noise gain becomes  
Mismatched feedback networks also result in a degradation of  
the ability of the circuit to reject input common-mode signals,  
much the same as for a four-resistor difference amplifier made  
from a conventional op amp.  
1
β
RF  
RG  
GN  
=
=1+  
Note that the output noise from VꢀCM goes to zero in this case.  
The total differential output noise density, vnꢀD, is the root-sum-  
square of the individual output noise terms.  
As a practical summarization of the previous issues, resistors of  
1% tolerance produce a worst-case input CMRR of approximately  
40 dB, a worst-case differential-mode output offset of 25 mV  
due to a 2.5 V VꢀCM input, negligible VꢀCM noise contribution,  
and no significant degradation in output balance error.  
8
vnOD  
=
v2  
nOi  
i=1  
Table 12 and Table 13 list several common gain settings, associated  
resistor values, input impedance, and output noise density for  
both balanced and unbalanced input configurations.  
CALCULATING THE INPUT IMPEDANCE FOR AN  
APPLICATION CIRCUIT  
The effective input impedance of a circuit depends on whether  
the amplifier is being driven by a single-ended or differential  
signal source. For balanced differential input signals, as shown  
in Figure 48, the input impedance (RIN, dm) between the inputs  
(+DIN and −DIN) is simply RIN, dm = RG + RG = 2 × RG.  
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.  
R
F
+V  
S
R
G
G
+IN  
+D  
–D  
IN  
V
OCM  
V
ADA4927  
The gain from the VꢀCM pin to Vꢀ, dm is equal to  
2(β1 − β2)/(β1 + β2)  
OUT, dm  
IN  
–IN  
R
–V  
S
When β1 = β2, this term goes to zero and there is no differential  
output voltage due to the voltage on the VꢀCM input (including  
noise). The extreme case occurs when one loop is open and the  
other has 100% feedback; in this case, the gain from VꢀCM input  
to Vꢀ, dm is either +2 or −2, depending on which loop is closed.  
R
F
Figure 48. The ADA4927 Configured for Balanced (Differential) Inputs  
Rev. 0 | Page 18 of 24  
 
 
 
 

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