MCP6021 datasheet(14/34 Pages) MICROCHIP

14 / 34 page

MCP6021/1R/2/3/4

DS21685C-page 14

After selecting RISO for your circuit, double-check the

resulting

frequency

response

peaking

and

step

response overshoot. Modify RISO’s value until the

response is reasonable. Evaluation on the bench and

simulations with the MCP6021/1R/2/3/4 Spice macro

model are helpful.

4.4

Gain Peaking

Figure 2-35 and Figure 2-36 use RF =1 kΩ to avoid

(frequency

response)

gain

peaking

and

(step

response) overshoot. The capacitance to ground at the

inverting input (CG) is the op amp’s common mode

input capacitance plus board parasitic capacitance. CG

is in parallel with RG, which causes an increase in gain

at high frequencies for non-inverting gains greater than

1 V/V (unity gain). CG also reduces the phase margin

of the feedback loop for both non-inverting and

inverting gains.

FIGURE 4-4:

Non-inverting gain circuit

with parasitic capacitance.

The largest value of RF in Figure 4-4 that should be

used is a function of noise gain (see GN in Section 4.3

“Capacitive Loads”) and CG. Figure 4-5 shows results

for various conditions. Other compensation techniques

may be used, but they tend to be more complicated to

the design.

FIGURE 4-5:

Non-inverting gain circuit

with parasitic capacitance.

4.5

MCP6023 Chip Select (CS)

The MCP6023 is a single amplifier with chip select

(CS). When CS is high, the supply current is less than

10 nA (typ) and travels from the CS pin to VSS, with the

amplifier output being put into a high-impedance state.

When CS is low, the amplifier is enabled. If CS is left

floating, the amplifier may not operate properly.

Figure 1-1 and Figure 2-39 show the output voltage

and supply current response to a CS pulse.

4.6

MCP6021 and MCP6023 Reference

Voltage

The single op amps (MCP6021 and MCP6023), not in

the SOT-23-5 package, have an internal mid-supply

reference voltage connected to the VREF pin (see

Figure 4-6). The MCP6021 has CS internally tied to

VSS, which always keeps the op amp on and always

provides a mid-supply reference. With the MCP6023,

taking the CS pin high conserves power by shutting

down both the op amp and the VREF circuitry. Taking

the CS pin low turns on the op amp and VREF circuitry.

FIGURE 4-6:

Simplified internal VREF

circuit (MCP6021 and MCP6023 only).

See Figure 4-7 for a non-inverting gain circuit using the

internal

mid-supply

reference.

The

DC-blocking

capacitor (CB) also reduces noise by coupling the op

amp input to the source.

FIGURE 4-7:

Non-inverting gain circuit

using VREF (MCP6021 and MCP6023 only).

VIN

VOUT

1.E+02

1.E+03

1.E+04

1.E+05

110

Noise Gain; GN (V/V)

GN > +1 V/V

100

10k

100k

CG = 7 pF

CG = 20 pF

CG = 50 pF

CG = 100 pF

VDD

VSS

VREF

50 k

(CS tied internally to VSS for MCP6021)

VIN

VOUT

VREF

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