MCP47DA1 datasheet(47/76 Pages) MICROCHIP

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 2012-2013 Microchip Technology Inc.

DS25118D-page 47

MCP47DA1

7.1.3

AMPLIFIER INPUT VOLTAGE (VW)

The input voltage into the Output Amplifier has require-

ments to ensure the input is in the linear range of the

amplifier.

To ensure that the amplifier is operating in its linear

range, the amplifier’s input voltage (VW) has some

requirements that must be met.

For device VDD voltages  3.0V, the amplifier is in the

linear region for all VREF voltages ( 1.0V) and DAC

For device VDD voltages < 3.0V, then the interaction

between the device VDD and the amplifier input voltage

(VW) need to be taken into account. The VW voltage is

dependent on the VREF voltage and the DAC register

code. Here is the amplifier requirement that must be

met:

VW  (VDD - 1.0V) / (2/3)

If VREF = VDD and VOUT will have full-scale output,

then:

VREF  (VDD - 1.0V) / (2/3)

Table 7-2 shows the maximum VREF voltage (for VDD <

3.0V) if the DAC output (VOUT) will operate over the full

range of DAC register codes.

TABLE 7-2:

VREF  VDD AND FULL-SCALE

OUTPUT

Table 7-3 shows the maximum DAC register code

when the VREF pin is tied to the VDD voltage (for VDD <

3.0V). For DAC register codes above this, the VOUT lin-

earity may be degraded (out of specification).

TABLE 7-3:

VREF = VDD AND NOT

FULL-SCALE OUTPUT

The VREF pin voltage and the maximum DAC register

code can be optimized between the maximum DAC

when the VREF voltage < VDD voltage < 3.0V, then the

DAC register code can be some value greater than the

code shown in Table 7-3. Figure 7-2 shows the equa-

tions for solving for VOUT voltage, the VREF voltage, or

the maximum DAC register code, based on knowing

the requirements for two of these variables. The DAC

number greater than 64 is invalid.

FIGURE 7-2:

Solving for VOUT, VREF, or

DAC Register Code.

VDD

VREF Comment

3.0

3.00

VREF pin can be tied to VDD pin

2.7

2.55

2.5

2.25

2.2

1.80

2.0

1.50

1.8

1.20

VDD =

VREF

Max DAC

Code

Comment

3.0

3.00

60h-FFh

This is Full Scale

2.7

2.55

58h

2.5

2.25

53h

2.2

1.80

48h

2.0

1.50

40h

1.8

1.20

35h

VOUT =

2 * VREF

DAC Code

VREF =

3 * VOUT

+ 1

DAC Code

DAC Code = 64 *

3 * VOUT

2 * VREF

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