ADA4932-1 Datasheet, PDF (22/28 Pages) Analog Devices – Low Power Differential ADC Driver

ADA4932-1/ADA4932-2
input terminals due to negative voltage feedback and is in phase
with the input signal, thus reducing the effective voltage across
R_G in the upper loop and partially bootstrapping R_G.
Terminating a Single-Ended Input
This section describes how to properly terminate a single-ended
input to the ADA4932-x with a gain of 1, R_F = 499 Ω, and R_G =
499 Ω. An example using an input source with a terminated output
voltage of 1 V p-p and source resistance of 50 Ω illustrates the four
steps that must be followed. Note that because the terminated
output voltage of the source is 1 V p-p, the open-circuit output
voltage of the source is 2 V p-p. The source shown in Figure 59
indicates this open-circuit voltage.
1. The input impedance is calculated using the formula

 

R_{IN ,se}
=




1−
R_G
R_F
2×(R_G +
R_F
)


=






1−
2×(
499
499
499 +


=
499)


665 Ω
V_S
2V p-p
R_{IN, se}
665Ω
R_S
R_G
50Ω
499Ω
V_OCM
R_G
499Ω
R_F
499Ω
+V_S
ADA4932-x
R_L V_{OUT, dm}
–V_S
R_F
499Ω
Figure 59. Calculating Single-Ended Input Impedance, R_IN
2. To match the 50 Ω source resistance, calculate the
termination resistor, R_T, using R_T||665 Ω = 50 Ω. The
closest standard 1% value for R_T is 53.6 Ω.
R_{IN, se}
50Ω
R_F
499Ω
+V_S
R_S
50Ω
V_S
2V p-p
R_T
53.6Ω
R_G
499Ω
V_OCM
R_G
499Ω
ADA4932-x
R_L V_{OUT, dm}
–V_S
R_F
499Ω
Figure 60. Adding Termination Resistor, R_T
3. Figure 60 shows that the effective R_G in the upper feedback
loop is now greater than the R_G in the lower loop due to the
addition of the termination resistors. To compensate for the
imbalance of the gain resistors, add a correction resistor (R_TS)
in series with R_G in the lower loop. R_TS is the Thevenin
equivalent of the source resistance, R_S, and the termination
resistance, R_T, and is equal to R_S||R_T.
R_S
50Ω
V_S
2V p-p
R_T
53.6Ω
V_TH
1.03V p-p
R_TH
25.9Ω
Figure 61. Calculating the Thevenin Equivalent
R_TS = R_TH = R_S||R_T = 25.9 Ω. Note that V_TH is greater than
1 V p-p, which was obtained with R_T = 50 Ω. The modified
circuit with the Thevenin equivalent (closest 1% value used for
R_TH) of the terminated source and R_TS in the lower feedback
loop is shown in Figure 62.
R_F
499Ω
+V_S
R_TH
R_G
V_TH
1.03V p-p
25.5Ω
499Ω
V_OCM
R_G
ADA4932-x
R_L ^VOUT, dm
R_TS 499Ω
25.5Ω
–V_S
R_F
499Ω
Figure 62. Thevenin Equivalent and Matched Gain Resistors
Figure 62 presents a tractable circuit with matched
feedback loops that can be easily evaluated.
It is useful to point out two effects that occur with a termi-
nated input. The first is that the value of R_G is increased in
both loops, lowering the overall closed-loop gain. The
second is that V_TH is a little larger than 1 V p-p, as it would
be if R_T = 50 Ω. These two effects have opposite impacts on
the output voltage, and for large resistor values in the feedback
loops (~1 kΩ), the effects essentially cancel each other out.
For small R_F and R_G, or high gains, however, the diminished
closed-loop gain is not canceled completely by the increased
V_TH. This can be seen by evaluating Figure 62.
The desired differential output in this example is 1 V p-p
because the terminated input signal was 1 V p-p and the
closed-loop gain = 1. The actual differential output voltage,
however, is equal to (1.03 V p-p)(499/524.5) = 0.98 V p-p.
To obtain the desired output voltage of 1 V p-p, a final gain
adjustment can be made by increasing R_F without modifying
any of the input circuitry. This is discussed in Step 4.
Rev. 0 | Page 22 of 28