Data Sheet

AD8432

Rev. D | Page 19 of 32

Table 5. Gain Setting Using a Pin-Strapping Technique and −3 dB Bandwidth for Each Gain Configuration

Differential

Gain (dB)

Single

Gain (dB)

−3 dB

BW (MHz)

RG1 (Ω)

RG2 (Ω)

RG3 (Ω)

RG4 (Ω)

RG5 (Ω)

RG6 (Ω)

RG7 (Ω)

12.04

6.02

200

12

12

Connect

GMH to GOH

Connect

GOH to OPH

24

Connect

GML to GOL

Connect

GOL to OPL

18.06

12.04

90

12

12

24

Connect

GOH to OPH

24

24

Connect

GOL to OPL

21.58

15.56

50

12

12

Connect

GMH to GOH

48

24

Connect

GML to GOL

48

24.08

18.06

32

12

12

24

48

24

24

48

The single-ended gain from INH to OPH (see Figure 65) is

defined as

G1

G4

G3

G2

G1

INH

OPH

R

R

R

R

R

G











The single-ended gain from INH to OPL is defined as

G1

G7

G6

G5

INH

OPL

R

R

R

R

G











The values of the seven gain resistors were chosen so that both

single-ended gains are equal. For example, to set a gain of

12.04 dB (G = ×4) differentially, the gain from INH to each

output (OPH, OPL) should be 6.02 dB (G = ×2).

2

2













G

G

G1

G2

G1

INH

OPH

R

R

R

R

R

G

2

2

















G

G

G1

G5

INH

OPL

R

R

R

R

G

ACTIVE INPUT RESISTANCE MATCHING

The AD8432 reduces noise and optimizes signal power transfer

by using active input termination to perform signal source

resistance matching.

The primary purpose of input impedance matching is to optimize

the input signal power transfer. With resistive termination, the

input noise increases due to the thermal noise of the terminating

resistor and the increased contribution of the input voltage

noise generator of the LNA. With active impedance matching,

however, the contributions of both are smaller than they are for

resistive termination by a factor of 1/(1 + ½ LNA) gain. The

noise figure (NF) for the three terminating schemes is shown in

Figure 67.

LNA

INH

UNTERMINATED

LNA

INH

RESISTIVE

TERMINATION

LNA

INH

ACTIVE

IMPEDANCE MATCH

Figure 66. Input Resistance Matching

To achieve this active impedance match, connect a feedback

given in Equation 1, where G/2 is the single-ended gain.

2

1

G

R

R

FB

IN





(1)

In addition, to further reduce the input resistance, there is an

internal resistance of 6.2 kΩ in parallel with the source resistance,

such that

INTERNAL

FB

IN

R

G

R

R

2

1 



(2)

input resistance and single-ended gain. Refer to Table 6 for

combinations.

kΩ

2

.

6

,

1

2

1















 





INTERNAL

INTERNAL

IN

IN

FB

R

R

R

G

R

R

(3)