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AN-815 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
AD8210 Bidirectional Operation Using a 3.3 V External Reference by Henri Sino
INTRODUCTION One of the key features of the AD8210 bidirectional current sense amplifier is the ability to offset the output to a voltage between 0 V (GND) to VS (supply voltage). This allows the user to monitor current flowing in both directions through the input shunt resistor while still operating with a single 5 V supply. Typically, bidirectional operation is achieved by tying one VREF pin to the supply while the other is tied to GND. In this mode, the output of the AD8210 starts at VS/2 and goes up or down depending on the direction of the current at its inputs. The AD8210 output can also be offset using an external reference with a voltage range of 0 V ≤ VREF ≤ VS. The configuration shown in Figure 1 makes use of the 3.3 V ADC supply and offsets the output of the AD8210 for bidirectional operation.
By tying the VREF1 pin to the A/D supply and the VREF2 pin to GND (see Figure 1), the output of the AD8210 is offset to half of the 3.3 V reference, or 1.65 V. It is important to note that while the output of the AD8210 can be referenced to any voltage within its supply range, the AD8210 supply voltage must remain 5 V. This means that regardless of the output offset voltage, the output range of the AD8210 must still be 0.05 V to 4.9 V. Calculations should be made so that the AD8210 output does not exceed the input range of the converter. Diode D1 is an optional component that can be used to protect the input of the converter from excessive overdriving. If D1 is used as shown in Figure 1, than the maximum voltage at the input of the converter would be a diode drop of 0.7 V (or 0.4 V if a Schottky diode is used) above the 3.3 V supply. Additionally, by using this external 3.3 V supply, the AD8210 can operate in unidirectional mode. This can be achieved by connecting both VREF pins to the external 3.3 V supply of the converter. In this case, the output of the AD8210 starts at 3.3 V.
I SHUNT
–IN
I
+IN
SHUNT
3.3V VREF 1
VREF 2
5V 1kΩ
+VS 0.1µF
NC
D1
–IN
+IN
ADC 400pF
OUT
3.3V VREF 1
GND
AD8210
Figure 1. Bidirectional Operation Using an External Reference
VREF 2
1kΩ
5V
ADC 400pF
+VS 0.1µF
NC
OUT
05714-002
AD8210
05714-001
GND
Figure 2. Unidirectional Operation Using an External Reference
When operating in this manner, the current through the input shunt resistor should flow from the negative input (Pin 1) to the positive input (Pin 8). As the current across the shunt resistor increases, the output correspondingly moves from 3.3 V down to 0.05 V.
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