TLC876M, TLC876I, TLC876C

10-BIT 20 MSPS PARALLEL OUTPUT CMOS

ANALOG-TO-DIGITAL CONVERTERS

SLAS140E – JULY 1997 – REVISED OCTOBER 2000

17

POST OFFICE BOX 655303

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.

10

µF

0.1

µF

10

µF

0.1

µF

140

Ω (± 1%)

NC

NC

REFTS

REFTF

REFBF

REFBS

250

Ω (± 1%)

5 V

4 V

2 V

TLC876

NC – No connect

500

Ω

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.