TLC876M Datasheet, PDF (13/23 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
driving the analog input (continued)
v RS
1
2f(CLK) (CE ln 2048)
– RSW
(1)
For f_(CLK) = 20 MHz, C_E = 10 pF, and R_SW = 100 Ω, this equation gives 228 Ω as a maximum value; hence the
200 Ω limit on the total source resistance. For applications with an input clock less than 20 MHz, the size of the
series resistor can increase proportionally. Alternatively, adding a shunt capacitor between the AIN terminal and
analog ground can lower the ac source impedance. This capacitance value depends on the source resistance
and the required signal bandwidth.
The input span is determined by the reference voltages (see driving the reference terminals section).
TLC876
AIN
S1
Driving
Source
RS
VS
RSW
CE
Ideal Source RS ≤ 200 Ω
TLC876
AIN
VS
Figure 12. TLC876 Simplified Equivalent Input
Figure 13. Sample TLC876 Drive Requirements
For many applications, particularly in single supply operation, ac coupling offers a convenient way of biasing
the analog input signal at the proper signal range. Figure 14 shows a typical configuration for ac coupling the
analog input signal to the TLC876. Maintaining the outlined specifications requires careful selection of the
component values. The most important concern is the f_{–3 dB} high-pass corner that is a function of R2, and the
parallel combination of C1 and C2. The f_{–3 dB} point can be approximated by equation 2.
* ^{+ f} 3 dB
2p
1
(R2) Ceq
(2)
where Ceq is the parallel combination of C1 and C2. Since C1 is typically a large electrolytic or tantalum
capacitor, the impedance becomes inductive at high frequencies. Adding a small ceramic or polystyrene
capacitor, C2 of approximately 0.01 µF, which is not inductive within the frequency range of interest, maintains
a low impedance. If the minimum expected input signal frequency is 20 kHz, and R2 equals 1 kΩ and R1 equals
50 Ω, the parallel capacitance of C1 and C2 must be a minimum of 0.008 µF to avoid attenuating signals close
to 20 kHz.
C1
VIN
C2
R1
R2
+
VBIAS
–
TLC876
AIN
Figure 14. AC-Coupled Inputs
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