MCP4716 datasheet(48/70 Pages) MICROCHIP

48 / 70 page

MCP47A1

DS25154A-page 48

 2012 Microchip Technology Inc.

8.1.1

DECREASING OUTPUT STEP SIZE

Due to the step voltage and output range of the

MCP47A1, it may be desirable to reduce the step

voltage while also modifying the range of the output. A

common method to achieve this smaller step size is a

voltage divider on the DAC’s output. Figure 8-2

illustrates this concept. Equation 8-2 shows a quick

estimation of the wiper value given the desired voltage

trip (VTRIP) point.

For example, if R1 = R2, then the VTRIP voltage range

is from VSS to 1/2 * VREF. Also at the VTRIP node, the

step voltage is 1/2 the step voltage at the VOUT node.

A bypass capacitor on the output of the voltage divider

plays a critical function in attenuating the output noise

of the DAC and the induced noise from

the

environment.

FIGURE 8-2:

Example Circuit Of Set Point

or Threshold Calibration.

EQUATION 8-2:

VOUT AND VTRIP

ESTIMATIONS

8.1.2

BUILDING A “WINDOW” DAC

When calibrating a set point or threshold of a sensor,

typically only a small portion of the DAC output range is

utilized. If the LSb size is adequate enough to meet the

application’s accuracy needs, the unused range is

sacrificed without consequences. If greater accuracy is

needed, then the output range will need to be reduced

to increase the resolution around the desired threshold.

If the threshold is not near VREF, 2 • VREF, or VSS then

creating a “window” around the threshold has several

advantages. One simple method to create this

“window” is to use a voltage divider network with a

pull-up and pull-down resistor. Figure 8-3 and Figure 8-

4 illustrate this concept.

FIGURE 8-3:

Single-Supply “Window”

DAC.

EQUATION 8-3:

VOUT AND VTRIP

ESTIMATIONS

Note:

The VOUT voltage can also be scaled by a

resistor from the VREF pin to the system

reference voltage. Care should be taken

with this implementation due to the ± 20%

variation to the 20k

 typical resistance

from the VREF pin to ground (RVREF). This

variation in resistance directly effects the

actual VOUT voltage.

VCC+

VCC–

I2C™

2-wire

VREF

MCP47A1

VDD

VOUT

Comp.

VTRIP

VSENSE

VOUT = N * VS

VS = VREF / 64

R1 + R2

VTRIP = VOUT *

VCC+

VCC–

I2C™

2-wire

VREF

MCP47A1

VDD

VOUT

VCC+

VCC–

RSENSE

Comp.

VTRIP

VOUT = N * VS

VS = VREF / 64

VOUT * R23 + V23 * R1

R1 + R23

VTRIP =

R23

V23

VOUT

VTRIP

Thevenin

Equivalent

R2 * R3

R2 + R3

R23 =

(VCC+ * R2) * (VCC- * R3)

R2 + R3

V23 =

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