TLC876M Datasheet, PDF (17/23 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
SLAS140D – JULY 1997 – REVISED MAY 2000
PRINCIPLES OF OPERATION
dc considerations (continued)
5
4.5
4
3.5
2 V Span
3
2.5
2
1 V Span
1.5
1
0.5
0
0 0.5 1 1.5 2 2.5 3 3.5 4
REFBF, REFBS
Figure 18. TLC876 Reference Ranges
ac considerations
The simplified diagram of Figure 17 shows that the reference terminals connect to a capacitor for one half of
the clock period. The size of the capacitor is a function of the analog input voltage, therefore producing dynamic
impedance changes at the reference inputs.
The external reference source must be able to maintain a low impedance over all frequencies of interest to
provide the charge required by the capacitance. By supplying the requisite charge, the reference voltages
remain relatively constant maintaining specified performance. For some reference configurations, voltage
transients are present on the reference lines, especially during the falling edge of CLK. The reference must
recover from the transients and settle to the desired level of accuracy prior to the rising edges of CLK.
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.
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