TLV5535

8-BIT, 35 MSPS, LOW-POWER ANALOG-TO-DIGITAL CONVERTER

SLAS221 – JUNE 1999

14

POST OFFICE BOX 655303

PRINCIPLE OF OPERATION

analog input (continued)

AIN

CI

S1

RSW

RS

VS

TLV5535

Figure 12. Simplified Equivalent Input Circuit

dc coupled input

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

R

2

V

REF

–V

IN

_

+

AVDD

VIN

REFTI

REFTO

REFBI

REFBO

AIN

TLV5535

_

+

VREF

VIN

REFTI

REFTO

REFBI

REFBO

AIN

TLV5535

RIN

R1

RIN

R2

(a)

(b)

Figure 13. DC-Coupled Input Circuit

ac coupled input

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

f

–3 dB +

1

÷ 2π xR

2

xCeq

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.