Texas Instruments TLV1544EVM, User Manual

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Circuit Function
3-3 Circuit Description
3.2 Circuit Function
The following paragraphs describe the function of individual circuits.
3.2.1 Inputs
The ADC has four analog inputs; A0, A1, A2, and A3. The EVM connects to
these inputs as follows:
-
A0. One op amp of a single-supply TLV2232 dual op amp drives the A0
input with an onboard-adjustable dc voltage.
-
A1. A TLE2027 dual-supply op amp connected as a voltage follower drives
the A1 input. The op amp receives external input through J2.
-
A2, A3. These are available for user-defined inputs. The external inputs
to these channels are applied through J3 and J4 respectively.
3.2.2 A0 – Voltage Variable Analog Input (Potentiometer)
Potentiometer R26 controls the dc voltage to ADC input A0 through one sec-
tion of the TLV2432, a non-inverting gain of 2 amplifier. Resistors R25 and R26
form a voltage divider from AV
CC
with the R26 wiper to the noninverting input.
Adjusting R26, the 10-k
Ω
potentiometer, through the adjustment range
changes the op amp input voltage from 0 to V
CC/2
. Since the amplifier has a
gain of two, the input voltage to A0 ranges from 0 to 5 volts. Measurements
made with this analog channel are ratiometric, since the input voltage varies
with changes in the supply voltage. The full scale output of the TLV2432 will
be approximately 10 counts below the nominal full scale digital output of all
ones if the analog V
CC
is used as the reference voltage input.
3.2.3 A1 – External Input With Voltage Follower Buffer
Input A1 of the TLC1544 input port connects through U5, a TLE2027 voltage
follower. J2 provides the external input for the voltage follower. With an input
power supply of
±
7 volts or greater, this amplifier configuration produces an
extremely linear signal from 0 to 5 volts because this signal range does not
approach the nonlinearity close to the supply rails. Input A1 lead is protected
from voltages above and below the ADC supply by diodes, D3 and D4. Diodes
D1 and D2 with resistor R29 protect the TLE2027 noninverting input from
voltages at J2 in excess of the supply rails. R30 is set to 0 ohms but can be
changed to provide additional current limit protection.
When using this amplifier with direct coupling, the noninverting input signal
should have a quiescent value of V
CC/2
for proper midpoint biasing for the
ADC. If capacitor coupling is used between the source and J2, then JP6 should
be removed so that the Vcc/2 reference can be established with an equal value
10-k resistor divider or the divider can be buffered with the unused op amp in
U4. Figure 3-2 shows this technique.