Datasheet

 2004 Microchip Technology Inc. DS21908A-page 27
MCP6S91/2/3
6.0 APPLICATIONS INFORMATION
6.1 Changing External Reference
Voltage
Figure 6-1 shows a MCP6S91 with the V
REF
pin at
2.5V and V
DD
= 5.0V. This allows the PGA to amplify
signals centered on 2.5V, instead of ground-referenced
signals. The voltage reference MCP1525 is buffered by
a MCP6021, which gives a low output impedance
reference voltage from DC to high frequencies. The
source driving the V
REF
pin should have an output
impedance less than 0.1Ω to maintain reasonable gain
accuracy.
FIGURE 6-1: PGA with Different External
Reference Voltage.
6.2 Capacitive Load and Stability
Large capacitive loads can cause stability problems
and reduced bandwidth for the MCP6S91/2/3 family of
PGAs (Figure 2-26 and Figure 2-28). As the load
capacitance increases, there is a corresponding
increase in frequency response peaking and step
response overshoot and ringing. This happens
because a large load capacitance decreases the
internal amplifier’s phase margin and bandwidth.
When driving large capacitive loads with these PGAs
(i.e., > 60 pF), a small series resistor at the output
(R
ISO
in Figure 6-2) improves the internal amplifier’s
stability by making the load resistive at higher
frequencies. The bandwidth will be generally lower
than the bandwidth with no capacitive load.
FIGURE 6-2: PGA Circuit for Large
Capacitive Loads.
Figure 6-3 gives recommended R
ISO
values for
different capacitive loads. After selecting R
ISO
for your
circuit, double-check the resulting frequency response
peaking and step response overshoot on the bench.
Modify R
ISO
’s value until the response is reasonable at
all gains.
FIGURE 6-3: Recommended R
ISO
.
6.3 Layout Considerations
Good PC board layout techniques will help achieve the
performance shown in the Electrical Characteristics
and Typical Performance Curves. It will also help
minimize Electromagnetic Compatibility (EMC) issues.
6.3.1 COMPONENT PLACEMENT
Separate different circuit functions: digital from analog,
low-speed from high-speed, and low-power from high-
power. This will reduce crosstalk.
Keep sensitive traces short and straight. Separate
them from interfering components and traces. This is
especially important for high-frequency (low rise time)
signals.
V
DD
V
DD
V
DD
V
REF
V
IN
V
OUT
MCP1525
MCP6021
1µF
MCP6S91
2.5V
REF
V
IN
V
OUT
MCP6S9X
R
ISO
C
L
10
100
1,000
10 100 1,000 10,000
L oad C apacitance (F)
R
ecommended
R
ISO
(
Ω
Ω
)
10 p 100 p 1 n 10 n