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WM2626ID

Low Power Dual 8-bit Serial Input DAC with Internal Reference

Wolfson Microelectronics plc 

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DEVICE DESCRIPTION

WM2626

GENERAL FUNCTION

The WM2626 is a dual 8-bit, voltage output DAC with an on-chip voltage reference. It uses a

resistor string network buffered with an op amp to convert 8-bit digital data to analogue voltage

levels (see Block Diagram). The output voltage is determined by the reference voltage and the

input code according to the following relationship:

( ) CODE

V = 2 VREFIN

out

256

INPUT

OUTPUT

1111

1111

:

( ) 255

2 VREF

256

:

1000

0001

( ) 129

2 VREF

256

1000

0000

( ) 128

2 VREF

= VREF

256

0111

1111

:

( ) 127

2 VREF

256

:

0000

0001

( )1

2 VREF

256

0000

0000

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

Before writing any data to the WM2626, the interface must be enabled by setting NCS to low.

Incoming data on DIN (starting with the MSB) is then shifted bit-per-bit into the internal register on

the falling edges of SCLK. From there data is loaded into the target latch after 16 bits have been

transferred, or when NCS rises. Four internal latches can be addressed: DAC A, DAC B, the buffer

latch or the control register. Their function is explained below (see ‘Register Addressing’).

SERIAL CLOCK AND UPDATE RATE

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

f SCLK max

=

1

tWH min + tWL min

= 20MHz

Since a data word contains 16 bits, the sample rate for one channel is limited to

( ) fs max

=

16 ×

1

tWH min

+ tWL min

= 1.25MHz

For full two-channel operation, where two data words need to be transmitted per sample, this figure

is halved to 625kHz. However, the DAC settling time to 8-bit accuracy limits the response time of

the analogue output for large input step transitions.

WOLFSON MICROELECTRONICS LTD

PD Rev 1.0 April 2001

7

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