9

The input bias current for any DC-coupled amplifier must

have an external path back to the amplifier’s power supply.

No such path exists in Figure 8A, and consequently the

amplifier output will remain in saturation.

A single large resistor (1M

Ω to 10MΩ) from either signal line

to power supply common will provide the required path, but a

resistor on each line is necessary to preserve accuracy. A

single pair of these bias current resistors on the HI-539

output may be used if their loading effect can be tolerated

pair on each input channel of the multiplexer is required.

The use of bias current resistors is acceptable only if one is

confident that the sum of signal plus common-mode voltage

will remain within the input range of the multiplexer/amplifier

combination.

Another solution is to simply run a third wire from the low

side of the signal source, as in Figure 8B. This wire assures

a low common-mode voltage as well as providing the path

for bias currents. Making the connection near the multiplexer

will save wire, but it will also unbalance the line and reduce

the amplifier's common-mode rejection.

Differential Offset,

There are two major sources of

with increasing temperature, as shown in the Electrical

Specifications tables. The other source of offset is the

thermocouple effects due to dissimilar materials in the signal

path. These include silicon, aluminum, tin, nickel-iron and

(often) gold, just to exit the package.

For the thermocouple effects in the package alone, the

constraint on

difference in temperature for package pins leading to any

produces a 5

µV offset. Obviously, this ∆T effect can

dominate the

∆V

care is taken in mounting the Hl-539 package.

Temperature gradients across the Hl-539 package should

be held to a minimum in critical applications. Locate the Hl-

539 far from heat producing components, with any air

currents flowing lengthwise across the package.

TABLE 1.

WIRE GAGE

EQUIVALENT WIDTH OF

P.C. CONDUCTOR

(2 oz. Cu)

DC RESISTANCE

PER FOOT

INDUCTANCE PER

FOOT

IMPEDANCE PER FOOT

60Hz

10kHz

18

0.47”

0.0064

Ω

0.36

µH

0.0064

Ω

0.0235

Ω

20

0.30”

0.0102

Ω

0.37

µH

0.0102

Ω

0.0254

Ω

22

0.19”

0.0161

Ω

0.37

µH

0.0161

Ω

0.0288

Ω

24

0.12”

0.0257

Ω

0.40

µH

0.0257

Ω

0.0345

Ω

26

0.075”

0.041

Ω

0.42

µH

0.041

Ω

0.0488

Ω

28

0.047”

0.066

Ω

0.45

µH

0.066

Ω

0.0718

Ω

30

0.029”

0.105

Ω

0.49

µH

0.105

Ω

0.110

Ω

32

0.018”

0.168

Ω

0.53

µH

0.168

Ω

0.171

Ω

HI-539