Data Sheet

AD8205

Rev.

| Page 11 of 12

APPLICATIONS INFORMATION

A typical application for the AD8205 is high-side measurement

of a current through a solenoid for PWM control of the

solenoid opening. Typical applications include hydraulic

transmission control and diesel injection control.

Two typical circuit configurations are used for this type of

application.

HIGH-SIDE CURRENT SENSE WITH A LOW-SIDE

SWITCH

In this case, the PWM control switch is ground referenced. An

inductive load (solenoid) is tied to a power supply. A resistive

shunt is placed between the switch and the load (see Figure 19).

An advantage of placing the shunt on the high side is that the

entire current, including the re-circulation current, can be

measured since the shunt remains in the loop when the switch

is off. In addition, diagnostics can be enhanced because shorts

to ground can be detected with the shunt on the high side.

In this circuit configuration, when the switch is closed, the

common-mode voltage moves down to near the negative rail.

When the switch is opened, the voltage reversal across the

inductive load causes the common-mode voltage to be held one

diode drop above the battery by the clamp diode.

–IN

INDUCTIVE

LOAD

AD8205

CLAMP

DIODE

42V

BATTERY

SHUNT

SWITCH

NC = NO CONNECT

5V

Figure 19. Low-Side Switch

HIGH-SIDE CURRENT SENSE WITH A HIGH-SIDE

SWITCH

This configuration minimizes the possibility of unexpected

solenoid activation and excessive corrosion (see Figure 20). In

this case, both the switch and the shunt are on the high side.

When the switch is off, this removes the battery from the load,

which prevents damage from potential shorts to ground, while

still allowing the recirculating current to be measured and

providing for diagnostics. Removing the power supply from the

load for the majority of the time minimizes the corrosive effects

that could be caused by the differential voltage between the load

and ground.

When using a high-side switch, the battery voltage is connected

to the load when the switch is closed, causing the common-

mode voltage to increase to the battery voltage. In this case,

when the switch is opened, the voltage reversal across the

inductive load causes the common-mode voltage to be held one

diode drop below ground by the clamp diode.

–IN

INDUCTIVE

LOAD

AD8205

CLAMP

DIODE

42V

BATTERY

SHUNT

SWITCH

NC = NO CONNECT

5V

Figure 20. High-Side Switch

Another typical application for the AD8205 is as part of the

control loop in H-bridge motor control. In this case, the

AD8205 is placed in the middle of the H-bridge (see Figure 21)

so that it can accurately measure current in both directions by

using the shunt available at the motor. This is a better solution

than a ground referenced op amp because ground is not

typically a stable reference voltage in this type of application.

This instability in the ground reference causes the

measurements that could be made with a simple ground

referenced op amp to be inaccurate.

The AD8205 measures current in both directions as the

H-bridge switches and the motor changes direction. The output

of the AD8205 is configured in an external reference

bidirectional mode, see the Output Offset Adjustment section.

–IN

AD8205

SHUNT

5V

2.5V

5V

CONTROLLER

NC = NO CONNECT

MOTOR

Figure 21. Motor Control Application

D