AD8338

Data Sheet

Rev. B | Page 16 of 19

EXPLANATION OF THE GAIN FUNCTION

The signal chain of the AD8338 can be broken down into three

stages. The first stage is a differential, voltage to current converter

input resistors can either be the internal 500 Ω resistors coupled

to Pin INPR and Pin INMR, or external resistors coupled to

Pin INPD and Pin INMD. The transresistance of the voltage to

the resistors is given by

N

P

INMx

INPx

IN

R

R

V

V

I

+

−

=

(8)

the AD8338, the VGA core. The VGA core is a fully differential

variable gain current amplifier with a gain range of 80 dB. In

the noninverting gain slope setting (Pin MODE connected to

Pin VBAT), the current gain of the VGA core spans from −26 dB

magnitude, the gain of the VGA core is given by

mV

250

20

/

)

34

80

(

_

10

02

.

0

10

GAIN

GAIN

V

IN

V

IN

VGA

OUT

I

I

I

×

×

≈

×

=

−

(9)

The differential output current of the VGA core is fed to the

third stage of the AD8338, a fully differential, current to voltage

converter comprised of the output amplifiers and their

such that the differential output voltage of the stage is given by

FBK

VGA

OUT

DIFF

OUT

R

I

V

×

×

=

2

,

_

(10)

Therefore, the overall voltage gain of the AD8338 is

( )

34

2

log

20

)

(

80

dB

−

















+

×

×

+

×

=

N

P

FBK

GAIN

R

R

R

V

G

(11)

Alternatively, the gain equation can be expressed as a numerical

gain magnitude:

mV

250

10

2

02

.

0

GAIN

V

N

P

FBK

N

R

R

R

G

×

+

×

×

=

(12)

Equation 11 and Equation 12 show that the gain range of the

to yield a range of 20 dB to 100 dB (see Figure 43). Similarly,

shifts the gain range down by 20 dB, to yield a range of −20 dB

to +60 dB.

As shown in Figure 43, when using external resistors to drive

the INPD and INMD pins, short the INPR and INMR pins to

one another to prevent stability issues.

Effects of Using External Resistors

When the gain range is shifted through the use of external

resistors, several trade-offs must be considered. External

resistors connected to Pin INPD and Pin INMD load the

current inputs of the VGA core, changing the dynamic behavior

of the block and the −3 dB bandwidth of the AD8338. The

−3 dB bandwidth of the AD8338 with external resistors is

Ω

×

















+

Ω

×

Ω

×

=

500

1

500

500

MHz

18

EXT

EXT

CL

R

R

f

(13)

For example, with 50 Ω external resistors, the input-referred

noise at maximum gain decreases to approximately 1 nV/√Hz,

and the gain range shifts up by 20 dB. However, the −3 dB

bandwidth is reduced from 18 MHz to approximately 1.8 MHz.