PC87591L datasheet(194/437 Pages) NSC

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Embedded Controller Modules (Continued)

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194

Revision 1.07

DAC Data Channel 0-3 Registers (DACDAT0-3)

These registers hold the data to be loaded into Channels 0-3 of the DAC. These registers are not affected by reset or disable

of the respective channel.

Location: Channel 0 - 00 FF4216

Channel 1 - 00 FF4416

Channel 2 - 00 FF4616

Channel 3 - 00 FF4816

Type: R/W

4.12.6 Usage Hints

Power Consumption

When a channel is enabled and no load is connected, the DAC current consumption depends on the value set in DACDAT

DAC Output Protection

To maintain the high performance of the analog circuits, the DAn (n=0 to 3) pins are not back-drive protected. Therefore, the

voltage applied to these pins must be within the AGND to AVCC range; otherwise the device may be damaged.

External circuits should not drive currents into these pins when the PC87591x is not powered up, because this may cause

the internal power-up reset circuit to fail.

Output Voltage Accuracy

Besides the intrinsic accuracy of the D/A channels, the output voltage accuracy directly depends on the accuracy of the

AVCC power supply, which serves as reference voltage. In order to improve the accuracy of the output voltage, the actual

AVCC value, measured by the ADC module (see Section 4.11 on page 170), should be used when computing the value of

DACDATA7-0 in DACDATn registers (see Section 4.12.3 on page 190).

The external load on DA0-3 pins may also affect the final output voltage of the DAC. Since the output resistance of these

pins is typically 3 K

Ω, use high-impedance loads; if high accuracy or high output currents are required, use external analog

drivers.

For the worst case calculation, if the output resistance is 4 K

Ω (maximum limit), the external load (R

L) must not be lower

than 2 M

Ω (see Figure 68). In this case, the error caused by the load is lower than 1/2 LSB.

To work with loads of 3 K

Ω (1 mA at 3V) with an error lower than 1/2 LSB, the output resistance of the external driver should

be lower than:

ROEXT <3 KΩ /(2*256) = 5.8 Ω

Bit

76543210

Name

DAC DATAi

Bit

Description

7-0

DAC Data. 8-bit unsigned binary value used for the D/A operation.

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