Microchip Technology MCP6S22 PGA PICtail, Руководство пользователя

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MCP6S2X Evaluation Board (Rev. 4) User’s Guide
DS51327B-page 12

2004 Microchip Technology Inc.
1.4.5 Reference Voltage
The MCP6S2X Evaluation Board (Rev. 4) has three reference-voltage settings, as
shown in Figure 1-3. The MCP1525, a precision reference voltage device, is used to
provide a 2.5V reference. The minimum supply voltage for this device is 2.7V.
Therefore, this device will not operate when the evaluation board supply voltage (V
DD
)
is below 2.7V. An adjustable reference voltage (0V to 5V) is also available using a
mechanical potentiometer. Since the reference voltage input of the PGA requires a
low-impedance source, these reference voltages are buffered using the MCP6022, a
10 MHz operational amplifier. The PGA reference voltage input can also be connected
to ground. All of these options can be selected using the on-board jumper connector
JP1 (refer to
Appendix D.”Setup Conditions” and D.5 “Reference Voltage Jumper
Positions” for more information).
FIGURE 1-3: Simplified Reference Voltage Schematic.
The input and output of this PGA can swing rail-to-rail (V
DD
and GND supply voltages).
However, in order to keep linearity, the output voltage should not exceed the Maximum
Output Voltage Swing, referred to in the electrical specifications of the MCP6S21/2/6/8
Data Sheet (DS21117). The reference voltage must be adjusted so that the maximum
output would not exceed the specified limit.
The PGA output voltage, with respect to the input and reference voltages, can be
determined using the following equation:
EQUATION 1-1:
V
REF
JP1
V
DD
1/2 MCP6022
V
DD
1/2 MCP6022
V
DD
V
DD
10 k
Ω
Pot.
+
+
-
-
MCP1525
Where: G
MCP6S26
= gain of 1, 2, 4, 5, 8, 10, 16 or 32 V/V
G
MCP6S21
= gain of 1, 2, 4, 5, 8, 10, 16 or 32 V/V
V
IN_X
= signal at the MCP6S26’s selected channel
V
REF
= Reference Voltage at V
REF
pin.
V
OUT_1
G
MCP6S26
V
IN_X
V
REF
–
( )
V
REF
+ =
V
OUT_2
G
MCP6S26
G
MCP6S21
×
V
IN_X
V
REF
–
( )
V
REF
+ =