參數(shù)資料
型號(hào)： TC7126ACKW713
元件分類： ADC
英文描述： 1-CH DUAL-SLOPE ADC, PQFP44
封裝： PLASTIC, QFP-44
文件頁(yè)數(shù)： 5/22頁(yè)
文件大小： 553K
代理商： TC7126ACKW713
2002 Microchip Technology Inc.
DS21458B-page 13
TC7126/A
7.2
Differential Reference
VREF+(Pin 36), VREF-(Pin 35)
The reference voltage can be generated anywhere
within the V+ to V- power supply range.
To prevent rollover type errors being induced by large
Common mode voltages, CREF should be large com-
pared to stray node capacitance.
The TC7126A offers a significantly improved analog
common temperature coefficient. This potential pro-
vides a very stable voltage, suitable for use as a refer-
ence. The temperature coefficient of analog common is
typically 35ppm/°C for the TC7126A and 80 ppm/°Cfor
the TC7126.
FIGURE 7-2:
COMMON MODE VOLTAGE REMOVED IN BATTERY OPERATION WITH
VIN = ANALOG COMMON
7.3
Analog Common(Pin32)
The analog common pin is set at a voltage potential
approximately 3V below V+. The potential is between
2.7V and 3.35V below V+. Analog common is tied inter-
nally to an N-channel FET capable of sinking 100
A.
This FET will hold the common line at 3V should an
external load attempt to pull the common line toward
V+. Analog common source current is limited to 1
A.
Therefore, analog common is easily pulled to a more
negative voltage (i.e., below V+ – 3V).
The TC7126A connects the internal VIN+and VIN-
inputs to analog common during the auto-zero phase.
During the reference integrate phase, VIN- is con-
nected to analog common. If VIN- is not externally con-
nected to analog common, a Common mode voltage
exists, but is rejected by the converter's 86dB Com-
mon mode rejection ratio. In battery operation, analog
common and VIN- are usually connected, removing
Common mode voltage concerns. In systems where
VIN- is connected to power supply ground or to a given
voltage, analog common should be connected to VIN-.
The analog common pin serves to set the analog sec-
tion reference, or common point. The TC7126A is spe-
cifically designed to operate from a battery, or in any
measurement system where input signals are not refer-
enced (float) with respect to the TC7126A's power
source. The analog common potential of V+ – 3V gives
a 7V end of battery life voltage. The common potential
has a 0.001%/% voltage coefficient and a 15
output
impedance.
With sufficiently high total supply voltage (V+ – V- > 7V),
analog common is a very stable potential with excellent
temperature stability (typically 35ppm/°C). This poten-
tial can be used to generate the TC7126A's reference
voltage. An external voltage reference will be unneces-
sary in most cases because of the 35ppm/°C tempera-
ture coefficient. See Section 7.5, TC7126A Internal
Voltage Reference discussion.
7.4
TEST (Pin 37)
The TEST pin potential is 5V less than V+. TEST may
be used as the negative power supply connection for
external CMOS logic. The TEST pin is tied to the inter-
nally generated negative logic supply through a 500
resistor. The TEST pin load should be no more than
1mA. See Section 5.0, Digital Section for additional
information on using TEST as a negative digital logic
supply.
If TEST is pulled HIGH (to V+), all segments plus the
minus sign will be activated. DO NOT OPERATE IN
THIS MODE FOR MORE THAN SEVERAL MINUTES.
With TEST = V+, the LCD segments are impressed
with a DC voltage which will destroy the LCD.
VBUFF
CAZ
VINT
BP
POL
Segment
Drive
OSC1
OSC3
OSC2
V-
V+
VREF+
VREF-
ANALOG
COMMON
V-
V+
V-
V+
GND
Measured
System
Power
Source
9V
LCD
TC7126A
+
VIN-
VIN+
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