MCP632 datasheet(23/60 Pages) MICROCHIP

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DS20002197C-page 23

MCP631/2/3/4/5/9

The largest value of RF that should be used depends

on the noise gain (see GN in Section 4.3.1

“Capacitive Loads”), CG and the open-loop gain’s

phase

shift.

Figure 4-9

shows

the

maximum

recommended RF for several CG values. Some

applications may modify these values to reduce either

output loading or gain peaking (step response

overshoot).

FIGURE 4-9:

Maximum Recommended

RF vs. Gain.

Figures 2-34 and 2-35 show the small signal and large

signal step responses at G = +1 V/V. The unity-gain

buffer usually has RF =0 and RG open.

Figures 2-36 and 2-37 show the small signal and large

signal step responses at G = -1 V/V. Since the noise

gain is 2 V/V and CG  10 pF, the resistors were

chosen to be RF =RG =1 k and RN = 500.

It is also possible to add a capacitor (CF) in parallel with

RF to compensate for the destabilizing effect of CG.

This makes it possible to use larger values of RF. The

conditions for stability are summarized in Equation 4-6.

EQUATION 4-6:

4.4

MCP633, MCP635 and MCP639

Chip Select

The MCP633 is a single amplifier with Chip Select

(CS). When CS is pulled high, the supply current drops

to 1 µA (typical) and flows through the CS pin to VSS.

When this happens, the amplifier output is put into a

high-impedance state. By pulling CS low, the amplifier

is enabled. The CS pin has an internal 5 M

 (typical)

pull-down resistor connected to VSS, so it will go low if

the CS pin is left floating. Figures 1-1, 2-42 and 2-43

show the output voltage and supply current response to

a CS pulse.

The MCP635 is a dual amplifier with two CS pins; CSA

controls op amp A and CSB controls op amp B. These

op amps are controlled independently, with an enabled

quiescent current (IQ) of 2.5 mA/amplifier (typical) and

a disabled IQ of 1 µA/amplifier (typical). The IQ seen at

the supply pins is the sum of the two op amps’ IQ; the

typical value for the IQ of the MCP635 will be 2 µA,

2.5 mA or 5 mA when there are 0, 1 or 2 amplifiers

enabled, respectively.

The MCP639 is a quad amplifier with two CS pins; CSB

controls op amp B and CSD controls op amp D.

4.5

Power Supply

With this family of operational amplifiers, the Power

Supply pin (VDD for single supply) should have a local

bypass capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm

for good high-frequency performance. Surface mount,

multilayer ceramic capacitors, or their equivalent,

should be used.

These op amps require a bulk capacitor (i.e., 2.2 µF or

larger) within 50 mm to provide large, slow currents.

Tantalum capacitors, or their equivalent, may be a good

choice. This bulk capacitor can be shared with other

nearby analog parts as long as crosstalk through the

supplies does not prove to be a problem.

1.E+02

1.E+03

1.E+04

1.E+05

100

Noise Gain; GN (V/V)

GN > +1 V/V

100

10k

100k

CG = 10 pF

CG = 32 pF

CG = 100 pF

CG = 320 pF

CG = 1 nF

We need:

Given:

-------

R

FCF

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

----------





GBWP

---------------,





GBWP

---------------,





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