Datasheet

© 2006 Microchip Technology Inc. DS22004B-page 23
MCP6G01/1R/1U/2/3/4
4.7 Unused Amplifiers
An unused amplifier in a quad package (MCP6G04)
should be configured as shown in Figure 4-8. This
circuit prevents the output from toggling and causing
crosstalk. Because the V
IN
pin looks like an open
circuit, the GSEL voltage is automatically set at V
DD
/2,
and the gain is 1 V/V. The output pin provides a
buffered V
DD
/2 voltage and minimizes the supply
current draw of the unused amplifier.
FIGURE 4-8: Unused Amplifiers.
4.8 Typical Applications
4.8.1 DRIVING THE GAIN SELECT PIN
WITH A MICROCONTROLLER GPIO
PIN
The circuit in Figure 4-9 uses a microcontroller GPIO
pin to drive the Gain Select input (GSEL). Setting the
GPIO pin to logic low, high-Z or logic high gives a GSEL
voltage of 0V, V
DD
/2 or V
DD
, respectively (G = 10, 1 or
50).
FIGURE 4-9: Driving the GSEL Pin.
The microcontroller’s GPIO pin cannot produce a
leakage current of more than ±1 µA for this circuit to
function properly. In noisy environments, a capacitor
may need to be added to the GPIO pin.
4.8.2 DRIVING THE GAIN SELECT PIN
WITH A PWM SIGNAL
The circuit in Figure 4-10 uses a PWM output on a PIC
microcontroller (100 kHz clock rate) to drive the Gain
Select input (GSEL). Setting the PWM duty cycle to
0%, 50% or 100% gives a GSEL voltage of 0V, V
DD
/2
or V
DD
, respectively (G = 10, 1 or 50).
FIGURE 4-10: Driving the GSEL Pin.
The PWM clock rate needs to be fast so it is easily
filtered and does not interfere with the desired signal,
and it needs to be slow enough for good accuracy and
low crosstalk. This filter reduces the ripple at the GSEL
pin to about 7 mV
P-P
at V
DD
= 5.0V. The 10% settling
time is about 200 µs; the filter limits how quickly the
gain can be changed. Scale the resistors and/or
capacitors for other clock rates, or for different ripple.
4.8.3 GAIN RANGING
Figure 4-11 shows a circuit that measures the current
I
X
. The circuit’s performance benefits from changing
the gain on the SGA. Just as a hand-held multimeter
uses different measurement ranges to obtain the best
results, this circuit makes it easy to set a high gain for
small signals and a low gain for large signals. As a
result, the required dynamic range at the SGA’s output
is less than at its input (by up to 34 dB).
FIGURE 4-11: Wide Dynamic Range
Current Measurement Circuit.
¼ MCP6G04
V
OUT
MCP6G0X
V
IN
GSEL
V
DD
V
OUT
MCP6G0XV
IN
GSEL
MCU
GPIO
Pin
V
DD
V
DD
V
OUT
MCP6G0XV
IN
GSEL
PIC MCU
PWM
Output
4.7 nF
V
DD
V
DD
10 kΩ
4.7 nF
10 kΩ
I
X
V
OUT
MCP6G0X
R
S