AD8338

Data Sheet

Rev. 0 | Page 14 of 16

ADJUSTING THE OUTPUT COMMON-MODE

VOLTAGE

As with any differential output, the output of the AD8338 is

a differential voltage that is centered about a common-mode

AD8338 is nominally set to 1.5 V using an internal reference

(see Figure 42).

9.5kΩ

9.5kΩ

VREF = 1.5V

FBKP

FBKM

Figure 42. Output Common-Mode Voltage Set to 1.5 V (Default Setting)

The output common-mode voltage of the AD8338 can be

adjusted to directly drive ADCs with various input common-

mode requirements. To adjust the output common-mode voltage,

add a resistor from each feedback node (FBKP and FBKM) to

either COMM or VBAT. Adding a resistor from each feedback

node to VBAT decreases the output common-mode voltage; add-

ing a resistor from each feedback node to COMM increases the

output common-mode voltage (see Figure 43 and Figure 44).

Table 5 and Table 6 provide examples of resistor values for

decreasing or increasing the output common-mode voltage.

Table 5. Resistor Values for Decreasing the Output

Common-Mode Voltage (Resistor Tied to VBAT)

VBAT (V)

Resistor Value (Ω)

Tied to

5.0

0.9

55,417

VBAT

3.3

0.9

28,500

VBAT

3.0

0.9

23,750

VBAT

Table 6. Resistor Values for Increasing the Output

Common-Mode Voltage (Resistor Tied to COMM)

VBAT (V)

Resistor Value (Ω)

Tied to

Any

1.8

47,500

COMM

Any

2.0

28,500

COMM

Any

2.5

14,250

COMM

9.5kΩ

R1

R2

9.5kΩ

VREF = 1.5V

FBKP

FBKM

VBAT

VBAT

OUTP = 1.5V –

(VBAT – 1.5V) × 9.5k

Ω

R1

OUTM = 1.5V –

(VBAT – 1.5V) × 9.5k

Ω

R2

Figure 43. Decreasing the Output Common-Mode Voltage (Resistors

Connected Between the FBKP and FBKM Pins to the VBAT Pin)

9.5kΩ

R1

R2

9.5kΩ

VREF = 1.5V

FBKP

FBKM

COMM

COMM

OUTP = 1.5V –

(0 – 1.5V) × 9.5k

Ω

R1

OUTM = 1.5V –

(0 – 1.5V) × 9.5k

Ω

R2

Figure 44. Increasing the Output Common-Mode Voltage (Resistors

Connected Between the FBKP and FBKM Pins to the COMM Pin)

The AD8338 uses its internal reference for all signal processing.

Therefore, although the output common-mode voltage can be

changed through the application of external resistors, the VREF

signal cannot be changed. For applications that require dc coupling

to an ADC, a differential amplifier must be used.