ADA4930-1/ADA4930-2

Data Sheet

Rev. D | Page 22 of 25

Input Common-Mode Adjustment with Resistors

The circuit shown in Figure 54 shows an alternatemethodto

bias the amplifierinputs,eliminating thedcsource.

ADA4930

RL VOUT, dm

+VS

–VS

3.3V

3.3V

RS

50Ω

RG1

301Ω

RG2

301Ω

RF2

301Ω

RF1

301Ω

RCM

VOCM

RCM

VS

VS

VSOURCE

2V p-p

RT

VIN

Figure 54. Single-Supply Biasing Scheme with Resistors

Define β1 = RP/RF1 and β2 = RN/RF2, where RP = RG1||RCM||RF1

and RN = RG2||RCM||RF2.

Set RF1 = RF2 = RF to maintain a balanced system, as shown.

Write a nodal equation at VP and solve for VP.













+

+

+

=

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

voltage from the Specifications section.For a 3.3 V supply, VP min

= 0.3 V.

Determine the minimum input voltage, VIN min at the output of

the source. Recognize that once properly terminated, the source

voltage is ½ of its open circuit value. Therefore, VIN min = −0.5 V.

Rearrange the VP equation forRCM













−

−

+

=

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.

2. RCM for VP min = 0.3 V and VIN min = −0.5 V. RCM = 9933 Ω.

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

RIN-SE = 399.35 Ω.

5. RT, RTH, and VTH. RT = 57.16 Ω, RTH = 26.67 Ω, and

VTH = 1.067 V.

6. The new values for RG1and RG2. RG2 = 160.55 Ω and

RG1 = 133.88 Ω.

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

8. The new value of RCM. RCM = 4759.63 Ω.

9. Repeat Step 3 through Step 8 until the values of RG1and RG2

remain constant between iterations. After four iterations,

the final circuit is shown in Figure 55.

ADA4930

RL VOUT, dm

+VS

–VS

RS

50Ω

RG1

142Ω

RG2

170Ω

RF2

301Ω

RF1

301Ω

RCM

1.87kΩ

RCM

1.87kΩ

VOCM

+VS

+VS

VS

2V p-p

RT

65.1Ω

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