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| 型號： | MCP3551E |
| 廠商： | Microchip Technology Inc. |
| 英文描述： | SENSOR COMP LOAD CELL 2LBF |
| 中文描述： | 低功耗，單通道22位Δ-Σ模數(shù)轉(zhuǎn)換器 |
| 文件頁數(shù)： | 15/30頁 |
| 文件大?。?/td> | 616K |
| 代理商： | MCP3551E |
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2005 Microchip Technology Inc.
DS21950B-page 15
MCP3551/3
4.5
Voltage Reference Input Pin
The MCP3551/3 devices accept a single-ended
external reference voltage, to be connected on the
V
REF
input pin. Internally, the reference voltage for the
ADC is a differential voltage with the non-inverting input
connected to the V
REF
pin and the inverting input
connected to the V
SS
pin. The value of the reference
voltage is V
REF
-
V
SS
and the common mode of the
reference is always (V
REF
- V
SS
)/2.
The MCP3551/3 devices accept a single-ended
reference voltage from 0.1V to V
DD.
The converter
output noise is dominated by thermal noise that is
independent of the reference voltage. Therefore, the
output noise is not significantly improved by lowering
the reference voltage at the V
REF
input pin. However, a
reduced reference voltage will significantly improve the
INL performance since the INL max error is
proportional to V
REF2
(see Figure 2-4).
The charge and discharge of the input capacitor create
dynamic input currents at the V
REF
input pin inversely
proportional to the sampling capacitor, which is a func-
tion of the input reference voltage. The typical value of
the single-ended input impedance is 2.4 M
Ω
, with
V
DD
= V
REF
= 5V. The DC leakage current caused by
the ESD input diodes, though on the order of 1 nA
typically, can cause additional gain error proportional to
the source resistance at the V
REF
pin.
4.6
Power-On Reset (POR)
The MCP3551/3 devices contain an internal Power-On
Reset (POR) circuit that monitors power supply voltage
V
DD
during operation. This circuit ensures correct
device start-up at system power-up and power-down
events. The POR has built-in hysteresis and a timer to
give a high degree of immunity to potential ripple and
noise on the power supplies, as well as to allow proper
settling of the power supply during power-up. A 0.1 μF
decoupling capacitor should be mounted as close as
possible to the V
DD
pin, providing additional transient
immunity.
The threshold voltage is set at 2.2V, with a tolerance of
approximately ±5%. If the supply voltage falls below
this threshold, the MCP3551/3 devices will be held in a
reset condition or in Shutdown mode. When the part is
in Shutdown mode, the power consumption is less than
1 μA. The typical hysteresis value is around 200 mV in
order to prevent reset during brown-out or other
glitches on the power supply.
Once a power-up event has occurred, the device must
require additional time before a conversion can take
place. During this time, all internal analog circuitry must
settle before the first conversion can occur. An internal
timer counts 32 internal clock periods before the
internal oscillator can provide clock to the conversion
process. This allows all internal analog circuitry to
settle to their proper operating point. This timing is
typically less than 300 μs, which is negligible compared
to one conversion time (72.7 ms for the MCP3551).
Figure 4-4 illustrates the conditions for a power-up and
power-down event under typical start-up conditions.
FIGURE 4-4:
Power-On Reset Operation.
4.7
Shutdown Mode
When not internally converting, the two modes of
operation for the MCP3551/3 devices are
the
Shutdown and Sleep modes. During Shutdown mode,
all internal analog circuitry, including the POR, is turned
off and the device consumes less than 1 μA. When
exiting Shutdown mode, the device must require addi-
tional time before a conversion can take place. During
this time, all internal analog circuitry must settle before
the first conversion can occur. An internal timer counts
32 internal clock periods before the internal oscillator
can provide clock to the conversion process. This
allows all internal analog circuitry to settle to their
proper operating point. This timing is typically less than
300 μs, which is negligible compared to one conversion
time (72.7 ms for MCP3551).
4.8
Sleep Mode
During Sleep mode, the device is not converting and is
awaiting data retrieval; the internal analog circuitry is
still running and the device typically consumes 10 μA.
In order to restart a conversion while in Sleep mode,
toggling CS to a logic-high (placing the part in Shut-
down mode) and then back to a logic-low will restart the
conversion. Sleep can only be entered in Single
Conversion mode. Once a conversion is complete in
Single Conversion mode, the device automatically
enters Sleep mode.
V
DD
2.2V
2.0V
0V
Reset
Normal
Operation
Reset
Start-up
Time
300 μs
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