8

LTC1606

Internal Voltage Reference

The LTC1606 has an on-chip, temperature compensated,

curvature corrected, bandgap reference, which is factory

trimmed to 2.50V. The full-scale range of the ADC is equal

to (

reference is connected to the input of a buffer (1.64x)

through a 4k resistor (see Figure 3). The input to the buffer

or the output of the reference is available at REF (Pin 3).

The internal reference can be overdriven with an external

reference if more accuracy is needed. The buffer output

drives the internal DAC and is available at CAP (Pin 4). The

CAP pin can be used to drive a steady DC load of less than

2mA. Driving an AC load is not recommended because it

can cause the performance of the converter to degrade.

For minimum code transition noise, the REF pin and the

CAP pin should each be decoupled with a capacitor to

filter wideband noise from the reference and the buffer

(2.2

µF tantalum for the REF pin and 10µF tantalum for the

CAP pin).

Offset and Gain Adjustments

The LTC1606 offset and full-scale errors have been trimmed

at the factory with the external resistors shown in Figure

4. This allows for external adjustment of offset and full

scale in applications where absolute accuracy is impor-

tant. See Figure 5 for the offset and gain trim circuit. First,

adjust the offset to zero by adjusting resistor R3. Apply an

input voltage of –152.6

µV (–0.5LSB) and adjust R3 so the

code is changing between 1111 1111 1111 1111 and 0000

0000 0000 0000. The gain error is trimmed by adjusting

resistor R4. An input voltage of 9.999542V (+FS – 1.5LSB)

is changing between 0111 1111 1111 1110 and 0111

1111 1111 1111. Figure 6 shows the bipolar transfer

characteristic of the LTC1606.

Figure 3. Internal or External Reference Source

+

5

4

3

2

1

10

µF

2.2

µF

33.2k

1%

±10V INPUT

200

Ω

1%

AGND1

REF

CAP

AGND2

LTC1606

1606 • F04

Figure 4.

±10V Input Without Trim

+

5

4

3

2

1

10

µF

2.2

µF

33.2k

1%

±10V INPUT

200

Ω

1%

AGND1

REF

CAP

AGND2

LTC1606

1606 • F05

576k

R4

50k

R3

50k

5V

Figure 5.

±10V Input with Offset and Gain Trim

INPUT VOLTAGE (V)

0V

–1

LSB

1606 • F06

011...111

011...110

000...001

000...000

100...000

100...001

111...110

1

LSB

BIPOLAR

ZERO

111...111

FS/2 – 1LSB

–FS/2

FS = 20V

1LSB = FS/65536

Figure 6. LTC1606 Bipolar Transfer Characteristics

–

+

1606 • F03

INTERNAL

CAPACITOR

DAC

BANDGAP

REFERENCE

4k

10

µF

CAP

(4.096V)

2.2

µF

REF

(2.5V)

4

3

R

0.64R