
MCP6V31/1U
DS25127A-page 22
2012 Microchip Technology Inc.
4.3.4
SOURCE RESISTANCES
The input bias currents have two significant
components; switching glitches that dominate at room
temperature and below, and input ESD diode leakage
currents that dominate at +85°C and above.
Make the resistances seen by the inputs small and
equal. This minimizes the output offset caused by the
input bias currents.
The inputs should see a resistance on the order of 10
to 1 k at high frequencies (i.e., above 1 MHz). This
helps minimize the impact of switching glitches, which
are very fast, on overall performance. In some cases, it
may be necessary to add resistors in series with the
inputs to achieve this improvement in performance.
Small input resistances may be needed for high gains.
Without them, parasitic capacitances might cause
positive feedback and instability.
4.3.5
SOURCE CAPACITANCE
The capacitances seen by the two inputs should be
small and matched. The internal switches connected to
the inputs dump charges on these capacitors; an offset
can be created if the capacitances do not match. Large
input capacitances and source resistances, together
with high gain, can lead to positive feedback and
instability.
4.3.6
CAPACITIVE LOADS
Driving large capacitive loads can cause stability
problems for voltage feedback op amps. As the load
capacitance increases, the feedback loop’s phase
margin decreases and the closed-loop bandwidth is
reduced. This produces gain peaking in the frequency
response, with overshoot and ringing in the step
response. These zero-drift op amps have a different
output impedance than most op amps, due to their
unique topology.
When driving a capacitive load with these op amps, a
improves the feedback loop’s phase margin (stability)
by making the output load resistive at higher
frequencies. The bandwidth will be generally lower
than the bandwidth with no capacitive load.
FIGURE 4-7:
Output Resistor, RISO,
Stabilizes Capacitive Loads.
different capacitive loads and gains. The x-axis is the
load capacitance (CL). The y-axis is the resistance
(RISO).
GN is the circuit’s noise gain. For non-inverting gains,
GN and the Signal Gain are equal. For inverting gains,
GN is 1+|Signal Gain| (e.g., -1 V/V gives GN = +2 V/V).
FIGURE 4-8:
Recommended RISO values
for Capacitive Loads.
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. Bench evaluation is helpful.
4.3.7
STABILIZING OUTPUT LOADS
This family of zero-drift op amps has an output
double zero when the gain is low. This can cause a
large phase shift in feedback networks that have low
impedance near the part’s bandwidth. This large phase
shift can cause stability problems.
(RL +RISO)||(RF +RG), where RISO is before the load
maintain performance; it should be at least 10 k.
FIGURE 4-9:
Output Load.
RISO
CL
VOUT
U1
MCP6V3X
1.E+03
1.E+04
m
mended
R
ISO
()
R
L||(RF + RG) 100 k
10k
1k
1.E+02
1.E-11
1.E-10
1.E-09
1.E-08
1.E-07
1.E-06
Reco
m
Capacitive Load (F)
G
N = 1
G
N = 10
G
N = 100
100
10p
100p
1n
10n
100n
1μ
RG
RF
VOUT
U1
MCP6V3X
RL
CL