Datasheet AD623 (Analog Devices) - 22

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AD623 Data Sheet Amplifying Signals with Low Common-Mode Voltage
equations, the maximum and minimum input common-mode Because the common-mode input range of the AD623 extends voltages are given by the following equations: 0.1 V below ground, it is possible to measure small differential VCMMAX = V+ − 0.7 V − VDIFF × Gain/2 signals that have low or no common-mode component. Figure 54 VCMMIN = V− − 0.590 V + VDIFF × Gain/2 shows a thermocouple application where one side of the J-type thermocouple is grounded. These equations can be rearranged to give the maximum possible differential voltage (positive or negative) for a particular common-
5V
mode voltage, gain, and power supply. Because the signals on
0.1µF
A1 and A2 can clip on either rail, the maximum differential voltage is the lesser of the two equations.
J-TYPE RG AD623 THERMOCOUPLE 1.02kΩ OUTPUT
|VDIFFMAX| = 2 (V+ − 0.7 V − VCM)/Gain
REF
|VDIFFMAX| = 2 (VCM − V− +0.590 V)/Gain
2V
053 778- However, the range on the differential input voltage range is 0 0 Figure 54. Amplifying Bipolar Signals with Low Common-Mode Voltage also constrained by the output swing. Therefore, the range of V Over a temperature range of −200°C to +200°C, the J-type thermo- DIFF may need to be lower according the following equation: couple delivers a voltage ranging from −7.890 mV to +10.777 mV. Input Range ≤ Available Output Swing/Gain A programmed gain on the AD623 of 100 (RG = 1.02 kΩ) and a For a bipolar input voltage with a common-mode voltage that is voltage on the REF pin of 2 V result in the output voltage ranging roughly half way between the rails, VDIFFMAX is half the value that from 1.110 V to 3.077 V relative to ground. the previous equations yield because the REF pin is at midsupply.
INPUT DIFFERENTIAL AND COMMON-MODE
Note that the available output swing is given for different supply
RANGE vs. SUPPLY AND GAIN
conditions in the Specifications section. Figure 55 shows a simplified block diagram of the AD623. The The equations can be rearranged to give the maximum gain for voltages at the outputs of Amplifier A1 and Amplifier A2 are a fixed set of input conditions. The maximum gain is the lesser given by of the two equations. V Gain A2 = VCM + VDIFF/2 + 0.6 V + VDIFF × RF/RG MAX = 2 (V+ − 0.7 V − VCM)/VDIFF = VCM + 0.6 V + VDIFF × Gain/2 GainMAX = 2 (VCM − V− +0.590 V)/VDIFF VA1 = VCM − VDIFF/2 + 0.6 V + VDIFF × RF/RG Again, it is recommended that the resulting gain times the input = VCM + 0.6 V − VDIFF × Gain/2 range is less than the available output swing. If this is not the case,
+VS
the maximum gain is given by
7
GainMAX = Available Output Swing/Input Range Also for bipolar inputs (that is, input range = 2 V
2 A1
DIFF), the
–IN
maximum gain is half the value yielded by the previous equations
4 RF 1 50kΩ – 50kΩ 50kΩ
because the REF pin must be at midsupply.
V –V DIFF S 2 +
The maximum gain and resulting output swing for different input
6 GAIN A3 OUTPUT R
conditions is given in Table 10. Output voltages are referenced to
G VCM RF
the voltage on the REF pin.
8 50kΩ 50kΩ 50kΩ 5 +V REF S
For the purposes of computation, it is necessary to break down the
– 7 VDIFF 2
input voltage into its differential and common-mode components.
+ A2
Therefore, when one of the inputs is grounded or at a fixed
3 +IN
voltage, the common-mode voltage changes as the differential
4
55 -0 voltage changes. Take the case of the thermocouple amplifier in
–VS
778 00 Figure 54. The inverting input on the AD623 is grounded; Figure 55. Simplified Block Diagram therefore, when the input voltage is −10 mV, the voltage on the The voltages on these internal nodes are critical in determining noninverting input is −10 mV. For the purpose of the signal whether the output voltage is clipped. The VA1 and VA2 voltages swing calculations, this input voltage must be composed of a can swing from approximately 10 mV above the negative supply common-mode voltage of −5 mV (that is, (+IN + −IN)/2) and (V− or ground) to within approximately 100 mV of the positive a differential input voltage of −10 mV (that is, +IN − −IN). rail before clipping occurs. Based on this and from the previous Rev. F | Page 22 of 26 Document Outline Features Applications General Description Functional Block Diagram Revision History Specifications Single Supply Dual Supplies Specifications Common to Dual and Single Supplies Absolute Maximum Ratings ESD Caution Pin Configuration and Function Descriptions Typical Performance Characteristics Theory of Operation Applications Information Basic Connection Gain Selection Reference Terminal Input and Output Offset Voltage Error Input Protection RF Interference Grounding Ground Returns for Input Bias Currents Output Buffering Single-Supply Data Acquisition System Amplifying Signals with Low Common-Mode Voltage Input Differential and Common-Mode Range vs. Supply and Gain Additional Information Evaluation Board General Description Outline Dimensions Ordering Guide