MCP433X datasheet(80/88 Pages) MICROCHIP

80 / 88 page

MCP433X/435X

DS22242A-page 80

 2010 Microchip Technology Inc.

Figure B-3 and Figure B-4 show the wiper resistance

for VDD voltages of 5.5, 3.0, 1.8 Volts. These graphs

show that as the resistor ladder wiper node voltage

(VWCn) approaches the VDD/2 voltage, the wiper

resistance increases. These graphs also show the

different resistance characteristics of the NMOS and

PMOS transistors that make up the wiper switch. This

is demonstrated by the wiper code resistance curve,

which does not mirror itself around the mid-scale code

(wiper code = 128).

So why is the RW graphs showing the maximum

resistance at about mid-scale (wiper code = 128) and

the RBW graphs showing the issue at code 160?

This requires understanding low-voltage transistor

characteristics as well as how the data was measured.

FIGURE B-3:

Wiper Resistance (RW) vs.

Wiper Code and Temperature

(VDD = 5.5V, IW = 900 UA; VDD = 3.0V,

IW = 480 µA).

FIGURE B-4:

Wiper Resistance (RW) vs.

Wiper Code and Temperature

(VDD = 1.8V, IW = 260 µA).

The method in which the data was collected is

important to understand. Figure B-5 shows the

technique that was used to measure the RBW and RW

resistance. In this technique Terminal A is floating and

Terminal B is connected to ground. A fixed current is

then forced into the wiper (IW) and the corresponding

wiper voltage (VW) is measured. Forcing a known

current through RBW (IW) and then measuring the

voltage difference between the wiper (VW) and

Terminal A (VA), the wiper resistance (RW) can be

calculated, see Figure B-5. Changes in IW current will

change the wiper voltage (VW). This may effect the

device’s wiper resistance (RW).

FIGURE B-5:

RBW and RW Measurement.

Figure B-6 shows a block diagram of the resistor

network where the RAB resistor is a series of 256 RS

resistors. These resistors are polysilicon devices. Each

wiper switch is an analog switch made up of an NMOS

and PMOS transistor. A more detailed figure of the

wiper switch is shown in Figure B-7. The wiper

resistance is influenced by the voltage on the wiper

switches nodes (VG, VW and VWCn). Temperature also

influences the characteristics of the wiper switch, see

Figure B-4.

The NMOS transistor and PMOS transistor have

different characteristics. These characteristics as well

as the wiper switch node voltages determine the RW

resistance at each wiper code. The variation of each

wiper switch’s characteristics in the resistor network is

greater then the variation of the RS resistors.

The voltage on the resistor network node (VWCn) is

dependent upon the wiper code selected and the

voltages applied to VA, VB and VW. The wiper switch VG

voltage to VW or VWCn voltage determines how strongly

the transistor is turned on. When the transistor is

weakly turned on the wiper resistance RW will be high.

When the transistor is strongly turned on, the wiper

resistance (RW) will be in the typical range.

100

120

140

160

180

200

220

128

192

256

Wiper Code

-40C @ 3.0V

+25C @ 3.0V

+85C @ 3.0V

+125C @ 3.0V

-40C @5.5V

+25C @ 5.5V

+85C @ 5.5V

+125C @ 5.5V

520

1020

1520

2020

128

192

256

Wiper Code

-40C @ 1.8V

+25C @ 1.8V

+85C @ 1.8V

+125C @ 1.8V

floating

RBW = VW/IW

RW = (VW-VA)/IW

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