5-18

Buffer Gain

At the end of the auto-zero interval, the instantaneous noise

voltage on the auto-zero capacitor is stored, and subtracts

from the input voltage while adding to the reference voltage

during the next cycle. The result is that this noise voltage

effectively is somewhat greater than the input noise voltage

of the buffer itself during integration. By introducing some

voltage gain into the buffer, the effect of the auto-zero noise

(referred to the input) can be reduced to the level of the

inherent buffer noise. This generally occurs with a buffer gain

of between 3 and 10. Further increase in buffer gain merely

increases the total offset to be handled by the auto-zero

loop, and reduces the available buffer and integrator swings,

without improving the noise performance of the system. The

circuit

recommended

for

doing

this

with

the

ICL8068/ICL7104 is shown in Figure 7. With careful layout,

the circuit shown can achieve effective input noise voltages

on the order of 1 to 2

µV, allowing full 16-bit use with full scale

inputs of a low as 150mV. Note that at this level, thermoelec-

tric EMFs between PC boards, IC pins, etc., due to local

temperature changes can be very troublesome. For further

discussion, see Application Note AN030.

ICL8052 vs ICL8068

The ICL8052 offers significantly lower input leakage currents

than the ICL8068, and may be found preferable in systems

with high input impedances. However, the ICL8068 has

substantially lower noise voltage, and for systems where

system noise is a limiting factor, particularly in low signal

level conditions, will give better performance.

TABLE 3. THREE-STATE BYTE FORMATS AND ENABLE PINS

CE/LD

HBEN

MBEN

LBEN

ICL7104-16

POL

O/R

B16

B15

B14

B13

B12

B11

B10

B9

B8

B7

B6

B5

B4

B3

B2

B1

HBEN

LBEN

ICL7104-14

POL

O/R

B14

B13

B12

B11

B10

B9

B8

B7

B6

B5

B4

B3

B2

B1

ICL8052/8068 WITH

ICL7104-16

ICL7104-14

UNIT

200

800

4000

100

4000

mV

Buffer Gain

10

1

1

10

1

V/V

100

43

200

47

180

k

Ω

0.33

0.33

0.33

0.1

0.1

µF

11111

µF

10

1

1

10

1

µF

100

400

2000

50

2000

mV

Resolution

3.1

12

61

6.1

244

µV

FIGURE 7. ADDING BUFFER GAIN TO ICL8068

A2

+

-

A3

+

-

INTEG.

COMP.

A1

+

-

BUFFER

14

11

9

INT OUT

-INT IN

BUF OUT

10

-1.2V

2

-15V

+5V

1

-15V

7

8

+15V

12

+INT IN

13

8068

INT.

REF.

6

3

+BUF IN

5

REF

OUT

5k

Ω

10k

Ω

300pF

COMP

OUT

100k

Ω

10-50K

TO ICL7104

ICL8052/ICL7104, ICL8068/ICL7104