AS1504 datasheet(10/17 Pages) AMSCO

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AS1504, AS1505

austriamicrosystems

Data Sheet

Programming The Output Voltage

Figure 15. Equivalent DAC Circuit

Programming The Output Voltage

The output voltage range is determined by the external reference connected to pins VREFH and VREFL (see Figure 15

on page 10 for a simplified diagram of the equivalent DAC circuit).

VREFL for the AS1504 is internally connected to GND and therefore cannot be offset. Pin VREFH can be tied to VDD and

pin VREFL can be tied to GND establishing a basic rail-to-rail output voltage programming range. Other output ranges

are established by the use of different external voltage references.

The programmed output voltage is determined as:

VOUT (Dx) = (Dx)/256 x (VREFH – VREFL) + VREFL

(EQ 2)

Where:

Dx is the data contained in the 8-bit DACx latch.

For example, when VREFH = +5V and VREFL = 0V the output voltages will be generated per the codes listed in Table 6.

Reference Inputs

The reference input pins (VREFH and VREFL) set the output voltage range of all eight DACs. For the AS1504, only pin

VREFH is available to establish a programmable full-scale output voltage.

Note:

The external reference voltage can be any value between 0 and VDD but must not exceed VDD.

The AS1505 uses pin VREFL to establish the zero-scale output voltage. Any voltage can be applied between 0 and

VDD. VREFL can be smaller or larger than VREFH since the DAC design uses fully bi-directional switches as shown in

Figure 15. The input resistance to the DAC has a code dependent variation that has a nominal worst case measured at

55h, which is approximately 2kΩ. When VREFH is greater than VREFL, the REFL reference must be able to sink current

out of the DAC ladder, while the REFH reference is sourcing current into the DAC ladder. The DAC design minimizes

reference glitch current, thus maintaining minimum interference between DAC channels during code changes.

Table 6. Output Voltages

Data Bits

VOUTx

Output State (VREFH = +5V, VREFL = 0V)

255

4.98V

Full-Scale

128

2.50V

Half-Scale (Mid-Scale Reset Value)

0.02V

1 LSB

0.00V

Zero-Scale

GND

DAC

VREFL

VREFH

LSB

MSB

P CH

N CH

To other DACs

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