Production Data

WM2331

WOLFSON MICROELECTRONICS LTD

PD Rev 1.4 April 2001

21

DRIVING THE INTERNAL REFERENCE BUFFER

DRIVING THE VREF PIN (DIFFERENTIAL MODE)

Figure 24 shows the equivalent load on the VREF pin when driving the internal reference buffer via

this pin (MODE = AVDD/2 and REFSENSE = AVDD).

VREF

REFSENSE=AVDD,

Mode = AVDD/2

+

-

(AVDD+VREF) / 4

R

IN

AVDD

AGND

14K

Figure 24 Equivalent Circuit of VREF

IN

REF

REF

R

AVDD

V

I

×

−

=

4

3

Tolerance on this current is

DRIVING THE INTERNAL REFERENCE BUFFER (TOP/BOTTOM MODE)

Figure 25 shows the loading on the REFTS and REFBS pins in Top/Bottom mode due to the internal

reference buffer. Note that the sample and hold circuit must also be driven via these pins, which

adds additional load (see Driving the Sample & Hold Reference Inputs, above).

Mode = AVDD

+

-

AVDD + (REFTS + REFBS) / 4

R

IN

AGND

14K

AVDD

REFTS

or

REFBS

Figure 25 Equivalent Circuit of Inputs to Internal Reference Buffer

The input currents are given by:

IN

INTS

R

REFBS

AVDD

REFTS

I

×

−

−

=

4

3

and

IN

INBS

R

REFTS

AVDD

REFBS

I

×

−

−

=

4

3

These currents must be provided by the sources on REFTS and REFBS in addition to the

requirements of driving the sample and hold.