9

®

ADS7831

FIGURE 4a. Circuit Diagram With External Hardware Trim.

FIGURE 4b. Circuit Diagram Without External Hardware Trim.

CALIBRATION

The ADS7831 can be trimmed in hardware or software. The

offset should be trimmed before the gain since the offset

directly affects the gain.

Hardware Calibration

To calibrate the offset and gain of the ADS7831, install the

proper resistors and potentiometers as shown in Figure 4a.

The calibration range is

±12mV for the offset and ±30mV

for full scale.

Potentiometer P

1

and resistor R

1

form the offset adjust

circuit and P

are not critical. R

the value shown. They can easily be made smaller to

provide increased adjustment range. Reducing these below

15% of the indicated values could begin to adversely affect

the operation of the converter.

P

tion. However, larger resistances will push the useful adjust-

ment range to the edges of the potentiometer. P

probably not exceed 20k

Ω and P2 100kΩ in order to main-

tain reasonable sensitivity.

Software Calibration or No Calibration

The ADS7831 does not require external resistors for its

basic operation. However, the component is designed to be

used with an external 50

Ω resistor on the input, and the

specifications apply to this condition. If this resistor is not

used, the only specification that will be affected is total

unadjusted error.

With the 50

Ω resistor, the nominal input voltage range is

±2.5V and the total unadjusted error is ±10LSBs guaran-

teed. Without the 50

Ω resistor, the nominal input voltage

range will be

±2.46V and the total unadjusted error is not

guaranteed. While it will typically be much less, the total

unadjusted error could be as high as

±20LSBs.

50

Ω

AGND2

CAP

REF

AGND1

V

IN

+

0.1µF

10µF

V

IN

NOTE: Use 1% metal film resistors. Trim offset at 0V first, then trim

gain at 2.5V.

FIGURE 3. Using CS to Control Conversion and Read Timing.

t

9

Hi-Z State

BUSY

CS

R/C

DATA

BUS

MODE

Acquire

Data Valid

HI Z State

Convert

t

7

t

2

t

6

t

3

t

4

t

11

t

5

Acquire

t

13

t

11

t

1

t

11

t

11

50

Ω

AGND2

CAP

REF

AGND1

V

IN

+

0.1µF

10µF

V

IN

+5V

R

1

20k

Ω

R

2

604k

Ω

–5V

P

1

5k

Ω

P

2

5k

Ω

+5V