MCP6S21 datasheet(16/43 Pages) MICROCHIP

16 / 43 page

MCP6S21/2/6/8

DS21117A-page 16

 2003 Microchip Technology Inc.

4.0

ANALOG FUNCTIONS

The MCP6S21/2/6/8 family of Programmable Gain

Amplifiers (PGA) are based on simple analog building

blocks (see Figure 4-1). Each of these blocks will be

explained in more detail in the following sub-sections.

FIGURE 4-1:

PGA Block Diagram.

4.1

Input MUX

The MCP6S21 has one input, the MCP6S22 and

MCP6S25 have two inputs, the MCP6S26 has six

inputs and the MCP6S28 has eight inputs (see

Figure 4-1).

For the lowest input current, float unused inputs. Tying

these pins to a voltage near the used channels also

works well. For simplicity, they can be tied to VSS or

VDD, but the input current may increase.

The one channel MCP6S21 has the lowest input bias

current, while the eight channel MCP6S28 has the

highest. There is about a 2:1 ratio in IB between these

parts.

4.2

Internal Op Amp

The internal op amp provides the right combination of

bandwidth, accuracy and flexibility.

4.2.1

COMPENSATION CAPACITORS

The internal op amp has three compensation capaci-

tors connected to a switching network. They are

selected to give good small signal bandwidth at high

gains, and good slew rate (full power bandwidth) at low

gains. The change in bandwidth as gain changes is

between 2 MHz and 12 MHz. Refer to Table 4-1 for

more information.

TABLE 4-1:

GAIN VS. INTERNAL COMPENSATION CAPACITOR

MCP6S21–One input (CH0), no SO pin

MCP6S22–Two inputs (CH0, CH1), VREF tied internally

to VSS, no SO pin

MCP6S26–Six inputs (CH0 to CH5)

MCP6S28–Eight inputs (CH0 to CH7)

VOUT

VREF

VDD

SCK

CH1

CH0

CH3

CH2

CH5

CH4

CH7

CH6

VSS

MUX

SPI™

Logic

POR

Gain

Switches

Gain

(V/V)

Internal

Compensation

Capacitor

Typical GBWP

(MHz)

Typical SR

(V/µs)

Typical FPBW

(MHz)

Typical BW

(MHz)

Large

4.0

0.30

Large

4.0

0.30

4Medium

0.70

5Medium

0.70

Medium

0.70

2.4

Medium

0.70

2.0

Small

1.6

Small

1.6

2.0

Note 1: FPBW is the Full Power Bandwidth. These numbers are based on VDD = 5.0V.

2: No changes in DC performance (e.g., VOS) accompany a change in compensation capacitor.

3: BW is the closed-loop, small signal -3 dB bandwidth.

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