TLC876M, TLC876I, TLC876C

10-BIT 20 MSPS PARALLEL OUTPUT CMOS

ANALOG-TO-DIGITAL CONVERTERS

SLAS140E – JULY 1997 – REVISED OCTOBER 2000

15

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

driving the analog input (continued)

1

7

NC

5

6

4

2

3

0.1

µF

NC

VCC

RL = 4.99 kΩ

AIN

TLC876

14.7 k

Ω

3 k

Ω

RIN = 4.99 kΩ

2V(PP)

REFBF

0 V dc

–

+

A†

† Amplifier A can be an AD817 or AD818 with terminal numbers as shown. The AD817 and AD818

are wide bandwidth single supply op-amps.

Figure 16. Bipolar Level Shift

driving the reference terminals

dc considerations

The TLC876 requires an external reference on terminals REFTF and REFBF and a resistor array, nominally

500

Ω, is connected between terminals REFTF and REFBF. A Kelvin connection, using the TLC876 reference

sense terminals REFTS and REFBS, minimizes voltage drops caused by external and internal wiring

resistance.

Figure 17 shows the equivalent input structure for the reference terminals. There is approximately 5

Ω of

resistance between both REFTF and REFBF terminals and the reference ladder. If the force-sense connections

are not used, the voltage drop across the 5-

Ω resistors results in a reduced voltage appearing across the ladder

resistance. This reduces the input span of the converter. Applying a slightly larger span between the REFTF

and REFBF terminals compensates for this error. Note that the temperature coefficients of the 5-

Ω resistors are

1350 ppm. The effects of temperature should be considered when a force-sense reference configuration is not

used.

5

Ω

5

Ω

RARRAY

500

Ω

C (Equivalent)

CLK

CLK

TLC876

REFTF

REFTS

REFBS

REFBF

DAC

10

Ω

5

Ω

5

Ω

5

Ω

10

Ω

5

Ω

5

Ω

5

Ω

Figure 17. TLC876 Equivalent Reference Structure