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WM2636

12-bit Serial Input Voltage Output DAC with Internal Reference

Wolfson Microelectronics plc 

WM2636

DEVICE DESCRIPTION

Production Data

GENERAL FUNCTION

The device uses a resistor string network buffered with an op amp to convert 12-bit digital data

to analogue voltage levels (see Block Diagram). The output voltage is determined by the

reference input voltage and the input code according to the following relationship:

( ) CODE

Output voltage = 2 VREF

4096

1111

INPUT

1111

1111

1000

:

0000

0001

1000

0000

0000

0111

1111

1111

0000

:

0000

0001

0000

0000

0000

OUTPUT

( ) 4095

2 VREF

4096

:

( ) 2049

2 VREF

4096

( ) 2048

2 VREF

= VREF

4096

( ) 2047

2 VREF

4096

:

( )1

2 VREF

4096

0V

Table 1 Binary Code Table (0V to 2VREF Output), Gain = 2

POWER ON RESET

An internal power-on-reset circuit resets the DAC register to all 0s on power-up.

BUFFER AMPLIFIER

The output buffer has a near rail-to-rail output with short circuit protection and can reliably drive

a 2kΩ load with a 100pF load capacitance.

SERIAL INTERFACE

Explanation of data transfer:

First, the device has to be enabled with NCS set to low. Then, a falling edge of FS starts

shifting the data bit-per-bit (starting with the MSB) to the internal register on the falling edges of

SCLK. After 16 bits have been transferred or FS rises, the content of the shift register is moved

to the DAC latch which updates the voltage output to the new level.

The serial interface of the device can be used in two basic modes:

• four wire (with chip select)

• three wire (without chip select)

Using chip select (four wire mode), it is possible to have more than one device connected to

the serial port of the data source (DSP or microcontroller). If there is no need to have more

than one device on the serial bus, then NCS can be tied low.

SERIAL CLOCK AND UPDATE RATE

Figure 1 shows the device timing. The maximum serial rate is:

fSCLKmax =

1

tWCH min+ tWCL min

=

20MHz

The digital update rate is limited to an 800ns period, or 1.25MHz frequency. However, the DAC

settling time to 12 bits limits the update rate for large input step transitions.

WOLFSON MICROELECTRONICS LTD

PD Rev 1.0 July 1999

7

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