9

1605fc

LTC1605

Internal Voltage Reference

The LTC1605 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 unity-gain buffer

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.

Figure 3. Internal or External Reference Source

S

S

–

+

1605 • F03

INTERNAL

CAPACITOR

DAC

BANDGAP

REFERENCE

4k

2.2

µF

CAP

(2.5V)

2.2

µF

REF

(2.5V)

4

3

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).

Offset and Gain Adjustments

The LTC1605 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 important. 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 –152.6mV (– 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 character-

istic of the LTC1605.

Figure 4.

±10V Input Without Trim

+

5

4

3

2

1

2.2

µF

2.2

µF

33.2k

1%

±10V INPUT

200

Ω

1%

AGND1

REF

CAP

AGND2

LTC1605

1605 • F04

Figure 5.

±10V Input with Offset and Gain Trim

INPUT VOLTAGE (V)

0V

–1

LSB

1605 • 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. LTC1605 Bipolar Transfer Characteristics

DC Performance

One way of measuring the transition noise associated with

a high resolution ADC is to use a technique where a DC

+

5

4

3

2

1

2.2

µF

2.2

µF

33.2k

1%

±10V INPUT

200

Ω

1%

AGND1

REF

CAP

AGND2

LTC1605

1605 • F05

576k

R4

50k

R3

50k

5V