tc7136a Microchip Technology Inc., tc7136a Datasheet - Page 12

• Manufacturer Part Number: tc7136a
• Description: Low Power 3-1/2 Digit Analog-to-digital Converter
• Manufacturer: Microchip Technology Inc.

TC7136/TC7136A
An exact expression for C
EQUATION 6-1:
C
rollover
recommended.
6.4
The input buffer amplifier and integrator are designed
with Class A output stages. The output stage idling cur-
rent is 6µA. The integrator and buffer can supply 1µA
drive currents with negligible linearity errors. R
chosen to remain in the output stage linear drive
region, but not so large that PC board leakage currents
induce errors. For a 200mV full scale, R
2V full scale requires 1.8MΩ (see Table 6-1).
TABLE 6-1:
DS21461B-page 12
Component
Where:
INT
Note:
Value
R
C
C
must have low dielectric absorption to minimize
F
V
R
V
INT
INT
AZ
OSC
FS
INT
INT
Integrating Resistor (R
error.
C
F
R
= Clock frequency at Pin 38
= Full scale input voltage
= Integrating resistor
= Desired full scale integrator output swing
OSC
INT
OSC
=
= 48kHz (3 reading per sec).
= 180kΩ, C
A
(4000)
Nominal Full Scale Voltage
0.047µF
200mV
0.47µF
180kΩ
polypropylene


INT
F
OSC
OSC
V
1
is:
INT
= 50pF.




R
V
INT
FS
INT
INT
capacitor
0.047µF
1.8MΩ


0.1µF
)
is 180kΩ. A
2V
INT
is
is
6.5
C
equation:
EQUATION 6-2:
Note that F
nal clock. The backplane drive signal is derived by
dividing F
To achieve maximum rejection of 60Hz noise pickup,
the signal integrate period should be a multiple of
60Hz. Oscillator frequencies of 240kHz, 120kHz,
80kHz, 60kHz, 40kHz, etc. should be selected. For
50Hz rejection, oscillator frequencies of 200kHz,
100kHz, 66-2/3kHz, 50kHz, 40kHz, etc. would be suit-
able. Note that 40kHz (2.5 readings per second) will
reject both 50Hz and 60Hz.
6.6
A full scale reading (2000 counts) requires the input
signal be twice the reference voltage.
In some applications, a scale factor other than unity
may exist between a transducer output voltage and the
required digital reading. Assume, for example, a pres-
sure transducer output for 2000 lb/in
than dividing the input voltage by two, the reference
voltage should be set to 200mV. This permits the trans-
ducer input to be used directly. The differential refer-
ence can also be used when a digital zero reading is
required, when V
in temperature measuring instrumentation. A compen-
sating offset voltage can be applied between analog
common and V
between V
Required Full Scale Voltage*
OSC
Note:
should be 50pF. R
Oscillator Components
Reference Voltage Selection
OSC
IN
OSC
*V
+ and analog common.
200mV
REF
by 800.
2V
IN
is ÷ 4 to generate the TC7136A's inter-
IN
-. The transducer output is connected
= 2V
is not equal to zero. This is common
F
REF.
OSC

2002 Microchip Technology Inc.
=
OSC
0.45
RC
is selected from the
2
is 400mV. Rather
100mV
V
1V
REF