
9
TC811
TC811-7 11/5/96
2001 Microchip Technology Inc.
DS21472A
3-1/2 Digit Analog-To-Digital Converter with
Hold and Differential Reference Inputs
Figure 6b.
Figure 7. Common-Mode Voltage Removed in Battery Operation With V
–
IN = Analog Common
facing with sensors such as load cells and temperature
sensors. The TC811 is ideally suited to applications in hand-
held multimeters, panel meters, and portable instrumenta-
tion. The reference voltage can be generated anywhere
within the V+ to V– power supply range.
To prevent rollover type errors from being induced by
large common-mode voltages, CREF should be large com-
pared to stray node capacitance. A 0.1
F capacitor is a
typical value.
The TC811 offers a significantly improved Analog Com-
mon temperature coefficient. This provides a very stable
voltage suitable for use as a voltage reference. The tem-
perature coefficient of Analog Common is typically
35ppm/
°C.
DEVICE PIN FUNCTIONAL DESCRIPTION
Differential Signal Inputs (VIN
+
(Pin 31),
VIN
– (Pin 30))
The TC811 is designed with true differential inputs and
accepts input signals within the input stage common mode
voltage range (VCM). The typical range is V+ – 1.0 to V– +
1.5V. Common-mode voltages are removed from the sys-
tem when the TC811 operates from a battery or floating
power source (isolated from measured system) and VIN
– is
connected to Analog Common. (see Figure 8)
In systems where common-mode voltages exist, the
86dB common-mode rejection ratio minimizes error. Com-
mon-mode voltages do, however, affect the integrator out-
put level. A worse case condition exists if a large positive
VCM exists in conjunction with a full-scale negative differen-
tial signal. The negative signal drives the integrator output
positive along with VCM (Figure 8). For such applications the
integrator output swing can be reduced below the recom-
mended 2.0V full-scale swing. The integrator output will
swing within 0.3V of V+ or V– without increased linearity
error.
Reference (V
+
REF (Pin 36), V
–
REF (Pin 33))
Unlike the ICL7116, the TC811 has a differential refer-
ence as well as the “hold” function. The differential reference
inputs permit ratiometric measurements and simplify inter-
Oscillator
Full-Scale Voltage (VFS)
Freq. (kHz)
200mV
2.0V
RINT
CINT
RINT
CINT
32.768
180k
0.068
F
1.8M
0.068
F
40
150k
0.068
F
1.5M
0.068
F
VBUF
CAZ VINT
BP
POL
SEGMENT
DRIVE
OSC1
OSC2
V
–
V
+
VREF
+
VREF
–
ANALOG
COMMON
V
–
V
+
V
–
V
+
GND
MEASURED
SYSTEM
POWER
SOURCE
9V
LCD DISPLAY
TC811
+
V
–
V
+
470k
V +
20M
V +
20 pF
10pF
40kHZ
Figure 8. Common-Mode Voltage Reduces Available
Integrator Swing. (VCOM ≠ VIN)
RI
+
–
VIN
V
C I
INTEGRATOR
VI =
[
VCM
VIN
–
INPUT
BUFFER
CI =
=
RI
INTEGRATION CAPACITOR
INTEGRATION RESISTOR
4000
f
INTEGRATION TIME
TI ==
Where:
VI
CM
OSC
–
+
–
+
TI
RI CI