MCP6S93 datasheet(19/40 Pages) MICROCHIP

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 2004 Microchip Technology Inc.

DS21908A-page 19

MCP6S91/2/3

4.2.3

RAIL-TO-RAIL OUTPUT

The maximum output voltage swing is the maximum

swing possible under a particular amplifier load current.

The amplifier load current is the sum of the external

load current (IOUT) and the current through the ladder

resistance (ILAD); see Figure 4-2.

EQUATION 4-1:

FIGURE 4-2:

Amplifier Load Current.

See Figure 2-21 for the typical output headroom

(VDD – VOH or VOL – VSS) as a function of amplifier

load current.

The specification table states the output can reach

within 60 mV of either supply rail when RL =10 kΩ and

VREF =VDD/2.

4.2.4

INPUT VOLTAGE AND PHASE

REVERSAL

The MCP6S91/2/3 amplifier family is designed with

CMOS input devices. It is designed to not exhibit phase

inversion when the input pins exceed the supply

voltages.

Figure 2-29

shows

input

voltage

exceeding both supplies with no resulting phase

inversion.

The maximum voltage that can be applied to the input

pins (CHx) is VSS –0.3V to VDD + 0.3V. Voltages on

the inputs that exceed this absolute maximum rating

can cause excessive current to flow into or out of the

input pins. Current beyond ±2 mA can cause possible

reliability problems. Applications that exceed this rating

must be externally limited with an input resistor, as

shown in Figure 4-3.

FIGURE 4-3:

RIN limits the current flow

into an input pin.

4.3

Resistor Ladder

The

resistor

ladder

shown

Figure 4-1

(RLAD =RF +RG) sets the gain. Placing the gain

switches in series with the inverting input reduces the

parasitic capacitance, distortion and gain mismatch.

RLAD is an additional load on the output of the PGA and

causes additional current draw from the supplies. It is

also a load (ZIN_REF) on the external circuitry driving

the VREF pin.

In Shutdown mode, RLAD is still attached to the VOUT

and VREF pins. Thus, these pins and the internal ampli-

fier’s inverting input are all connected through RLAD

and the output is not High-Z (unlike the internal op

amp).

While RLAD contributes to the output noise, its effect is

small. Refer to Figure 2-12.

Where:

Amplifier Load Current

OUT

LAD

OUT

REF

–

()

LAD

-------------------------------------

VOUT

VREF

RLAD

IOUT

ILAD

MCP6S9X

CHx

RIN

VIN

RIN ≥

VSS – (Maximum expected VIN)

2 mA

RIN ≥

(Maximum expected VIN) – VDD

2 mA

VOUT

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