TLV5580

8BIT, 80 MSPS LOW POWER A/D CONVERTER

SLAS205B − DECEMBER 1998 − REVISED OCTOBER 2003

www.ti.com

15

PRINCIPLE OF OPERATION

DC COUPLED INPUT

_

+

AVDD

VIN

REFTI

REFTO

REFBI

REFBO

AIN

TLV5580

_

+

VREF

VIN

REFTI

REFTO

REFBI

REFBO

AIN

TLV5580

RIN

R1

RIN

R2

(a)

(b)

Figure 13. DC-Coupled Input Circuit

For dc-coupled systems an op amp can level-shift a ground-referenced input signal. A circuit as shown in

Figure 13(a) is acceptable. Alternatively, the user might want a bipolar shift together with the bottom reference

voltage as seen in Figure 13(b). In this case the AIN voltage is given by:

AIN

+ 2

R

2 ÷

R

1 )

R

2

V

REF

–V

IN

AC COUPLED INPUT

VIN

AIN

TLV5580

R1

R2

VBIAS

+

−

C1

C2

Figure 14. AC-Coupled Input Circuit

For many applications, especially in single supply operation, ac coupling offers a convenient way for biasing

the analog input signal at the proper signal range. Figure 14 shows a typical configuration. To maintain the

outlined specifications, the component values need to be carefully selected. The most important issue is the

positioning of the 3 dB high-pass corner point f−3 dB, which is a function of R2 and the parallel combination of

C1 and C2, called Ceq. This is given by the following equation:

f

–3 dB +

1

÷ 2π xR

2

xCeq

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 higher

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 low impedance. If the minimum expected input signal

frequency is 20 kHz, and R2 equals 1 k

Ω and R1 equals 50 Ω, the parallel capacitance of C1 and C2 must be

a minimum of 8 nF to avoid attenuating signals close to 20 kHz.