TLC876M_07 Datasheet, PDF (17/22 Pages) Texas Instruments – 10-BIT 20 MSPS PARALLEL OUTPUT CMOS ANALOG-TO-DIGITAL CONVERTERS

TLC876M, TLC876I, TLC876C
10-BIT 20 MSPS PARALLEL OUTPUT CMOS
ANALOG-TO-DIGITAL CONVERTERS
SLAS140E – JULY 1997 – REVISED OCTOBER 2000
PRINCIPLES OF OPERATION
ac considerations (continued)
Several useful reference configurations can be used depending on the application, desired level of accuracy,
and cost tradeoffs. The simplest configuration, shown in Figure 19, utilizes a resistor divider to generate the
reference voltages from the converters analog power supply. The 0.1 µF bypass capacitors reduce high
frequency transients. The 10 µF capacitors reduce the impedances at the REFTF and REFBF terminals at lower
frequencies; however, as input frequencies approach dc, the capacitors become ineffective, and small voltage
deviations appear across the biasing resistors. This reference method maintains 10-bit accuracy for input
frequencies above approximately 200 Hz and 8-bit accuracy applications for input frequencies above
approximately 50 Hz.
140 Ω (± 1%)
5V
10 µF
TLC876
NC
REFTS
4 V _REFTF
0.1 µF
500 Ω
250 Ω (± 1%)
10 µF
2V
REFBF
0.1 µF
NC
REFBS
NC – No connect
Figure 19. Low Cost Reference Circuit
The reference configuration in Figure 19 provides the lowest cost, but the disadvantages include reduced dc
power supply rejection and reduced accuracy due to the variability of the internal and external resistors.
The force-sense reference connections can eliminate the voltage drops associated with the internal
connections to the reference ladder. Figure 20 shows a circuit using a dual, rail-to-rail single-supply operational
amplifier. The operational amplifier should provide stable 3.6 V and 1.6 V reference voltages. Each half of the
amplifier is compensated to drive 1 µF and 0.1 µF decoupling capacitors at the REFTF and REFBF terminals
maintaining stability. The operational amplifiers are connected as voltage followers.
By connecting the operational amplifier feedback through the sense connections of the TLC876, the outputs
of the operational amplifiers automatically adjust to compensate for the voltage drops that occur within the
converter.
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