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LTC2641/LTC2642

26412f

OPERATION

The digital-to-analog transfer function at the VOUT pin

is:

OUT IDEAL

REF

() =

⎛

⎝⎜

⎞

⎠⎟

where k is the decimal equivalent of the binary DAC input

code, N is the resolution, and VREF is between 2.0V and

VDD (see Tables 1a, 1b and 1c).

The LTC2642 includes matched resistors that are tied to

an external ampliﬁer to provide bipolar output swing (Fig-

ure 2). The bipolar transfer function at the RFB pin is:

OUT BIPOLAR IDEAL

REF

)

–

⎛

⎝⎜

⎞

⎠⎟

(see Tables 2a, 2b and 2c).

Serial Interface

The LTC2641/LTC2642 communicates via a standard

3-wire SPI/QSPI/MICROWIRE compatible interface. The

chip select input (

⎯C⎯S) controls and frames the loading

of serial data from the data input (DIN). Following a

⎯C⎯S

high-to-low transition, the data on DIN is loaded, MSB

ﬁrst, into the shift register on each rising edge of the serial

clock input (SCLK). After 16 data bits have been loaded

into the serial input register, a low-to-high transition on

⎯C⎯S

transfers the data to the 16-bit DAC latch, updating the DAC

output (see Figures 1a, 1b, 1c). While

⎯C⎯S remains high,

the serial input shift register is disabled. If there are less

than 16 low-to-high transitions on SCLK while

⎯C⎯S remains

low, the data will be corrupted, and must be reloaded.

Also, if there are more than 16 low-to-high transitions

on SCLK while

⎯C⎯S remains low, only the last 16 data bits

loaded from DIN will be transferred to the DAC latch. For

the 14-bit DACs, (LTC2641-14/LTC2642-14), the MSB

remains in the same (left-justiﬁed) position in the input

16-bit data word. Therefore, two “don’t-care” bits must

be loaded after the LSB, to make up the required 16 data

bits (Figure 1b). Similarly, for the 12-bit family members

(LTC2641-12/LTC2642-12) four “don’t-care” bits must

follow the LSB (Figure 1c).

D15

MSB

SCLK

DIN

D14

D13

D12

D11

D10

DATA (16 BITS)

DAC

UPDATED

LSB

26412 F01a

D13

MSB

SCLK

DIN

D12

D11

D10

DATA (14 BITS + 2 DON’T-CARE BITS)

DAC

UPDATED

LSB

26412 F01b

MSB

SCLK

DIN

D11

D10

DATA (12 BITS + 4 DON’T-CARE BITS)

DAC

UPDATED

LSB

26412 F01c

Figure 1c. 12-Bit Timing Diagram (LTC2641-12/LTC2642-12)

Figure 1b. 14-Bit Timing Diagram (LTC2641-14/LTC2642-14)

Figure 1a. 16-Bit Timing Diagram (LTC2641-16/LTC2642-16)

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