AD3531 datasheet(29/44 Pages) AD

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Data Sheet

AD3531/AD3531R

APPLICATIONS INFORMATION

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Rev. 0 | 29 of 44

Option 6 (Controlled Update, Same Data,

Software LDAC, and Multiple Channel)

Option 6 allows immediate and simultaneous

update of multiple DAC_CHn registers identi-

fied by MULTI_INPUT_SEL_0(MI_SEL_CH_n) and

SW_LDAC_EN_0(SLD_EN_CH_n) bitfields with the same data.

Data is contained in MULTI_INPUT_CH register and is initiated

through the software LDAC.

Option 6 is ideal for multiple channel updates in both single instruc-

tion and stream modes.

Figure 79. Option 6 Example: Write to Multi_Input_Ch Register with a Soft-

ware LDAC, Stream Mode (Loop) and Address Ascending

Option 6 is the same as Option 5, except a software LDAC

function is used instead of a hardware LDAC function. The

SW_LDAC_EN_0 register determines which channel is affected

by a software LDAC command.

HEADROOM AND FOOTROOM

Headroom and footroom refers to the voltage difference between

the supply voltage and the intended output voltage of the DAC for

a specified output load current. If the supply voltage headroom or

footroom is insufficient, the pass element of the integrated output

amplifier of the DAC acts like a resistor instead of an ideal switch.

This causes the output voltage to drop as the load current increas-

es.

The AD3531/AD3531R has a very low typical headroom require-

ment of 25mV/20mA and 50mV/20mA for footroom. The typical

performance is shown in Figure 20. The voltage drop is generally

linear in nature, hence, can be calculated by multiplying the load

current by the headroom/footroom specification. For example, we

have a 5V supply and a 5V setting at the output of the DAC. If

the DAC starts sourcing current of 30mA to a load, the DAC output

voltage would be around 4.963V.

For a footroom example, a DAC output that is sinking 30mA of

current would result to an output voltage that is 75mV above ground

potential.

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