Datasheet > ADA4945-1CP-EBZ

ADA4945-1CP-EBZ

更新时间： 2022-02-26 10:59:43

品牌	Logo	应用领域
亚德诺 - ADI		/
页数	文件大小	规格书
44页	1219K
描述
High Speed Offset Drift Fully Differential ADC Driver

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ADA4945-1CP-EBZ 数据手册

ADA4945-1

Data Sheet

For an unbalanced single-ended input signal, as shown in

Figure 104, the input impedance is

IMPACT OF MISMATCHES IN THE FEEDBACK

NETWORKS

Even if the external feedback networks (R_F/R_G) 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.

β1+ β2

β1(β2 +1)

_IN,SE= R_G1

where:

R_G1

_G1+ R_F1

β1 =

As well as causing a noise contribution from V_OCM, ratio matching

errors in the external resistors result in a degradation of the ability

of the circuit to reject input common-mode signals, similar to a

four resistors difference amplifier made from a conventional

op amp.

R_G2

R_G2+ R_F2

β2 =

In addition, if the dc levels of the input and output common-

mode voltages are different, matching errors result in a small

differential mode, output offset voltage. When G = 1, with a

ground referenced input signal and the output common-mode

level set to 2.5 V, an output offset of as much as 25 mV (1% of

the difference in common-mode levels) can result if 1% tolerance

resistors are used. Resistors of 1% tolerance result in a worst

case input CMRR of about 40 dB, a worst case differential mode

output offset of 25 mV due to the 2.5 V level shift, and no

significant degradation in output balance error.

IN, SE

OCM

ADA4945-1

OUT, dm

–V

Figure 104. ADA4945-1 with Unbalanced (Single-Ended) Input

For a balanced system where R_G1= R_G2= R_Gand R_F1= R_F2= R_F,

the equations simplify to

CALCULATING THE INPUT IMPEDANCE OF AN

APPLICATION CIRCUIT









The effective input impedance depends on whether the signal

source is single-ended or differential. For a balanced differential

R_G

R_F

2(R_G+ R_F)









β1 = β2 =

and R_IN,SE=

R_G+ R_F

input signal, as shown in Figure 103, the input impedance (R_{IN, dm}

)

1−

between the inputs (+D_INand −D_IN) is R_{IN, dm}= 2 × R_G.

The input impedance of the circuit is effectively higher than it

would be for a conventional op amp connected as an inverter

because a fraction of the differential output voltage appears at

the inputs as a common-mode signal, partially bootstrapping

the voltage across the R_G1input resistor.

ADA4945-1

+IN

–D

OCM

OUT, dm

–IN

Figure 103. ADA4945-1 Configured for Balanced (Differential) Inputs

Rev. 0 | Page 38 of 44

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