8

Application Information

General

The Hl-539 accepts inputs in the range -15V to +15V, with

performance guaranteed over the

±10V range. At these

higher levels of analog input voltage it is comparable to the

HI-509, and is plug-in compatible with that device (as well as

the Hl-509A). However, as mentioned earlier, the Hl-539 was

designed to introduce minimum error when switching low level

inputs.

Special care is required in working with these low level

signals. The main concern with signals below 100mV is that

noise, offset voltage, and other aberrations can represent a

large percentage error. A shielded differential signal path is

essential to maintain a noise level below 50

µV

Low Level Signal Transmission

The transmission cable carrying the transducer signal is critical

in a low level system. It should be as short as practical and

rigidly supported. Signal conductors should be tightly twisted for

minimum enclosed area to guard against pickup of

electromagnetic interference, and the twisted pair should be

shielded against capacitively coupled (electrostatic)

interference. A braided wire shield may be satisfactory, but a

leakage capacitance to ground per foot. A key requirement for

the transmission cable is that it presents a balanced line to

sources of noise interference. This means an equal series

impedance in each conductor plus an equally distributed

impedance from each conductor to ground. The result should

be signals equal in magnitude but opposite in phase at any

transverse plane. Noise will be coupled in phase to both

conductors, and may be rejected as common-mode voltage by

a differential amplifier connected to the multiplexer output.

Coaxial cable is not suitable for low level signals because the

two conductors (center and shield) are unbalanced. Also,

ground loops are produced if the shield is grounded at both

ends by standard BNC connectors. If coax must be used, carry

the signal on the center conductors of two equal-length cables

whose shields are terminated only at the transducer end. As a

general rule, terminate (ground) the shield at one end only,

preferably at the end with greatest noise interference. This is

usually the transducer end for both high and low level signals.

Watch Small

∆V Errors

Printed circuit traces and short lengths of wire can add

substantial error to a signal even after it has traveled

hundreds of feet and arrived on a circuit board. Here, the

small voltage drops due to current flow through connections

of a few milliohms must be considered, especially to meet an

accuracy requirement of 12 bits or more.

Table 1 is a useful collection of data for calculating the effect

of these short connections. (Proximity to a ground plane will

lower the values of inductance.)

As an example, suppose the Hl-539 is feeding a 12-bit

converter system with an allowable error of

(

±1.22mV). lf the interface logic draws 100mA from the 5V

supply, this current will produce 1.28mV across 6 inches of

#24 wire; more than the error budget. Obviously, this digital

current must not be routed through any portion of the analog

ground return network.

FIGURE 7A. SINGLE-ENDED CROSSTALK TEST CIRCUIT

FIGURE 7B. DIFFERENTIAL CROSSTALK TEST CIRCUIT

FIGURE 7. CROSSTALK

HI-539

INSTRUMENTATION

AMPLIFIER

†

+

-

G = 1000

350

Ω

1kHz,

HI-539

INSTRUMENTATION

AMPLIFIER

†

+

-

G = 1000

350

Ω

1kHz,

350

Ω

HI-539