ADA4930-1/ADA4930-2

Data Sheet

Rev. C | Page 22 of 25

Input Common-Mode Adjustment with Resistors

The circuit shown in Figure 54 shows an alternate method to

bias the amplifier inputs, eliminating the dc source.

ADA4930

3.3V

3.3V

50Ω

301Ω

301Ω

301Ω

301Ω

2V p-p

Figure 54. Single-Supply Biasing Scheme with Resistors













+

+

+

=

CM

F

S

OCM

IN

G

F

P

R

R

V

V

V

R

R

β2

β1

β1β2

V

2

2

1

Determine VP min. This is the minimum input common-mode

= 0.3 V.

the source. Recognize that once properly terminated, the source













−

−

+

=

OCM

min

IN

G

F

P

F

S

CM

V

V

R

R

V

β1β2

β2

β1

R

V

R

2

2

1

1

1

min

Calculate the following:

1. β1 and β2. For the circuit shown in Figure 54, β1 = 0.5 and

β2 = 0.5.

3. The new values for β1 and β2. β1 = 0.4925 and β2 = 0.4925.

4. The input impedance using the following:

























−

+

+

=

























−

=

−

β1β2

R

R

β2

β1

β2

β1

R

V

V

R

R

G1

F1

G1

INP

P

G1

SE

IN

1

1

7. The new values for β1 and β2. β1 = 0.284 and β2 = 0.317.

remain constant between iterations. After four iterations,

the final circuit is shown in Figure 55.

ADA4930

50Ω

142Ω

170Ω

301Ω

301Ω

1.87kΩ

1.87kΩ

2V p-p

65.1Ω

Figure 55. Single-Supply, Single-Ended Input System with Bias Resistors