Circuit Note

CN-0582

analog.com

Rev. A | 4 of 10

Figure 2. IEPE Sensor Interface

IEPE Excitation and Control

IEPE sensors require power in the form of a constant current

source to operate properly. Aside from the constant current ranging

from 2 mA to 20 mA, the source must have a compliance of 24 V

to 30 V, which is at least twice the typical IEPE bias voltage of 8

V to 12 V. Figure 3 shows the IEPE high voltage, constant current

source topology based on the LT3092.

Figure 3. IEPE Current Source

Using the formula shown in Equation 1, the constant current source

for the CN0582 is set to 4 mA.

IOUT=10μA × RSET

ROUT

=10μA × 120kΩ

300Ω

=4mA (1)

where:

IOUT is the constant current output.

ROUT is the current output set resistance.

RSET is the current input set resistance.

The current source can be enabled or disabled by an ADG5401, a

high voltage, low resistance switch.

Commands sent from the software to the board toggle the corre-

sponding channel to the desired state.

Powering the ADG5401 from the same 26 V supply as the current

sources ensures that the input voltage remains within limits, while

the 6.5 Ω on-resistance is small enough to have an effect on the

signal level.

When the current source is disabled, the input for each channel

acts as a typical high impedance voltage input analog-to-digital

converter (ADC) measurement. Channel 3 features an on-board

load resistor that allows the user to connect and measure the data

from a 4 mA to 20 mA sensor, whose value can be computed using

Equation 2:

VSENSOR=RLOAD×ISENSOR

(2)

where:

VSENSOR is the converted sensor voltage.

RLOAD is the load resistor (249 Ω).

ISENSOR is the sensor output current.

Input Protection

To avoid overdriving any part of the input circuitry, transient voltage

suppressor (TVS) diodes are added at the input connector. These

diodes ensure that the input can withstand voltages up to 40

V, protecting the sensitive analog input circuitry from dangerous

voltage levels.

AC/DC Coupling

Depending on the sensor, either ac-coupling or dc-coupling may

be more appropriate. DC-coupling is applicable for sensors with

a response that extends to zero Hz, and that have a small, or

well-specified offset voltage. AC-coupling is more appropriate for

sensors that inherently have a highpass response, and may have a

large dc offset that varies over operating conditions.

Figure 4 shows the ac- and dc-coupled paths, controlled by an

ADG5436 high voltage, latch-up proof, dual SPDT switch.

Figure 4. AC or DC-Coupling Path Based on ADG5436

The switch is powered from a bipolar -5 V or 26 V supply, which

accommodates typical IEPE sensor outputs, as well as voltage

output sensors with 0 V to 5 V, ±5 V, and 0 V to 10 V ranges. The

ac-coupling path is composed of a high pass filter, whose cutoff

frequency can be calculated using Equation 3:

fc= 1

2π×499kΩ×22μF

=0.014Hz

(3)

where:

fc is the high pass cutoff frequency.