SP7800ADS/02

SP7800A 12-Bit 3

µs Sampling A/D Converter

© Copyright 2000 Sipex Corporation

7

INPUT VOLTAGE RANGE AND LSB VALUES

Input Voltage Range Defined As:

±10V

±5V

Analog Input Connected to Pin

1

2

Pin Connected to GND

2

1

One Least Significant Bit (LSB)

4.88mV

2.44mV

OUTPUT TRANSITION VALUES

FFEH TO FFFH

+ FULL SCALE

+10V–3/2LSB

+5V–3/2LSB

+9.9927V

+4.9963V

7FFH TO 800H

Mid Scale

0V–1/2LSB

0V–1/2LSB

(Bipolar Zero)

–2.44mV

–1.22mV

000H to 001H

–Full Scale

–10V+1/2LSB

–5V+1/2LSB

-9.9976V

-4.9988V

Table 2. Input Voltages, Transition Voltages and LSB Values

±5V Range Offset and Gain

Offset — Apply 1.5637V to the

±5V input at pin

2. Adjust the offset potentiometer until the LSB

toggles on and off at code 1010 1000 0000

A80

Gain — Apply 4.9963V to the

±5V input at pin

2. Adjust the gain potentiometer until the LSB

toggles on and off at code 1111 1111 1110

FFE

±10V Range Offset and Gain

Offset — Apply 1.2622V to the

±10V input at

pin 1. Adjust the offset potentiometer until the

LSB toggles on and off at code 100100000010

BIN

= 902

Gain — Apply 9.9927V to the

±10V input at pin

1. Adjust the gain potentiometer until the LSB

Figure 3. a)

±10V Range b) ±5V Range — With External Trims

toggles on and off at code 1111 1111 1110

FFE

Layout Considerations

Because of the high resolution and linearity of the

SP7800A, system design problems such as ground

path resistance and contact resistance become very

important.

The input resistance of the SP7800A is 6.3k

Ω or

4.2K

Ω (for the ±10V and ±5V ranges respectively).

To avoid introducing distortion, the source resistance

must be very low, or constant with signal level. The

output impedance provided by most op amps is ideal.

Pins23(V

on the SP7800A, to maximize accuracy on the chip.

They should be connected together as close as pos-

sible to the unit. Pin 24 may be slightly more sensitive

than pin 23 to supply variations, but to maintain

SP7800A

1

2

3

4

5

6

7

±10V

Input

GAIN ADJUST

100

Ω

10K

Ω

BIPOLAR ZERO ADJUST

499

Ω∗

∗ These resistors are different than those used with the ADS7800 Application Note.

These values will work with both the SP7800A and ADS7800.

–15V

+5V

SP7800A

1

2

3

4

5

6

7

GAIN ADJUST

±5V

Input

+5V

–15V

1K

Ω∗

30.1K

Ω

301

Ω

a)

b)