MCP6271R Microchip Technology Inc., MCP6271R Datasheet - Page 4

• Manufacturer Part Number: MCP6271R
• Description: 170 ?a, 2 Mhz Rail-to-rail Op Amp
• Manufacturer: Microchip Technology Inc.

Bypass Capacitors: No Black Magic Here
By Bonnie C. Baker, Microchip Technology Inc.
A basic requirement for all electronic circuits is the inclusion of
bypass capacitors (also know as decoupling capacitors). These
devices reside across the positive supply to ground, as close
to the supply pin of the active device as possible. You may get
away with excluding these capacitors in low-frequency circuits,
but more than not, low-frequency circuits actually have high-
frequency entities inside the active devices. An example of a
supposed “low-frequency device” is a microcontroller that uses a
low-frequency system clock. Granted, the frequency of the clock
is slow, but the transition of the internal and I/O gates can occur
in nanoseconds. Without proper power-supply filtering, these
rising and falling glitches will traverse the circuit. The first step
to proper supply filtering is to include a proper-valued, bypass
capacitor.
Circuits containing digital devices are not the only systems that
require bypass capacitors. Analog devices also benefit from the
inclusion of bypass capacitors, but in another way. While bypass
capacitors in digital systems control fast rising- and falling-time
glitches from the device, bypass capacitors in analog systems
assist in preventing power-supply noise from entering the analog
device. Typically, analog devices have preventative power-supply
filtering built-in, primarily known as power-supply or line-rejection
capability. These power-supply rejection (PSR), noise-rejection
mechanisms are effective at reducing low-frequency power-supply
noise, but this is not the case at higher frequencies.
Once you resign to your fate of including bypass capacitors
in your circuit, the re-aiming task is to select the right
capacitor value for the various devices in your circuit. Typically,
manufacturers will include suggested bypass capacitor values
in their data sheets. If the manufacturer does not supply that
information, you can determine the proper value on your own.
For instance, with microcontrollers or microprocessors you
can calculate the bypass capacitor value when you know the
typical rise and fall times of signals from your device (t
You also need the average current while the microcontroller or
microprocessor is operating (I
the microcontroller/microprocessor product data sheet tables.
You finally need to define the maximum voltage ripple-noise that
you can tolerate on your power-supply trace (V
Figure 1: Using the PSR versus over temperature of an analog part (A) in combination with the ceramic capacitor impedance versus
frequency (B) determines the best bypass capacitor for analog parts.
2
Analog and Interface Guide – Volume 2
Techniques To Minimize Noise
-20
-40
-60
-80
0
1k
A) 12-bit A/D Converter
AVE
10k
). These two quantities are in
Frequency (Hz)
100k
RIPPLE
1M
).
RISE
10M
).
100k
100
10k
Using these values, the appropriate value for the bypass
capacitor is:
Analog devices are a benefit in another way. With these kinds
of circuits, you need to find the frequency where power-supply
noise will affect your circuit. The best place to look to find this
information is with the PSR or line-rejection performance over
frequency graphs in the product data sheet. Additionally, you
need to determine the minimum, acceptable noise that you
can tolerate. For instance, with a 12-bit A/D converter you can
tolerate un-rejected power-supply noise of approximately ± ¼
Least Significant Bytes (LSB), for true 12-bit performance. You
also need to take a stab at estimating the power-supply, noise-
voltage magnitude. With these two parameters, you can refer
to the typical PSR versus frequency curve in the manufacturer’s
data sheet.
For example, Figure 1A provides the PSR over frequency curve of
a 12-bit A/D converter. The PSR of this converter is equal to:
If you determine that the noise level riding on top of your
converter power supply is ±20 mV (or 40 mV peak) and the
allowed error is ± ¼ LSB or 0.61 mV peak (implying 5V full scale
range), the noise from the power supply will show up in the
output code of the converter as noise as –36.33 dB. This occurs
per Figure 1A at approximately 5 MHz. Referring to Figure 1B, the
appropriate bypass, ceramic capacitor value for this converter
would be between 0.1 μF and 0.01 μF.
It is advantageous to note that these calculations use typical
values instead of the minimum or maximum numbers. This
is important to understand, because selecting the correct
bypass capacitor is not an exact science. Not only will devices
vary slightly from part to part, the capacitors that you use
in your circuit will also vary from part to part as well as over
temperature. But, don’t let this deter you from using bypass
capacitors. The worst of all cases is when you use none.
1M
10
1k
1
100
f
I
C
bypass capacitor value
PSR = 20 log (V
noise
SURGE
BYPASS
0.1 μf
Ceramic
= 1/( 2 * t
1k
B) Capacitor Response
= I
= I
AVE
SURGE
Frequency (Hz)
* f
10k
1 nf
Ceramic
noise
POWER
rise
/ ( 2 * π * f
100k
); Determine noise frequency
/ f
-
SUPPLY
micro
1M
-
; Approximate surge current
RIPPLE
noise
0.01 μf
Ceramic
* V
/ V
10M
ADC
RIPPLE
-
ERROR
); Calculate
)