Datasheet LT6106 (Analog Devices) - 8

LT6106
APPLICATIONS INFORMATION
by orders of magnitude. A sense resistor with integrated This approach can be helpful in cases where occasional Kelvin sense terminals will give the best results. Figure 2 bursts of high currents can be ignored. illustrates the recommended method. Care should be taken when designing the board layout for V+ RIN, especially for small RIN values. All trace and inter- connect resistances will increase the effective RIN value, RIN causing a gain error. RSENSE +IN –IN + –
Selection of External Output Resistor, ROUT
LOAD V+ V– The output resistor, ROUT, determines how the output cur- rent is converted to voltage. VOUT is simply IOUT • ROUT. OUT In choosing an output resistor, the maximum output volt- LT6106 VOUT age must first be considered. If the following circuit is a ROUT buffer or ADC with limited input range, then R 6106 F02 OUT must be chosen so that IOUT(MAX) • ROUT is less than the allowed
Figure 2. Kelvin Input Connection Preserves Accuracy with
maximum input range of this circuit.
Large Load Currents
In addition, the output impedance is determined by ROUT. If the circuit to be driven has high enough input impedance,
Selection of External Input Resistor, RIN
then almost any useful output impedance will be accept- RIN should be chosen to allow the required resolution able. However, if the driven circuit has relatively low input while limiting the output current to 1mA. In addition, the impedance, or draws spikes of current such as an ADC might maximum value for RIN is 500Ω. By setting RIN such that do, then a lower ROUT value may be required in order to the largest expected sense voltage gives IOUT = 1mA, then preserve the accuracy of the output. As an example, if the the maximum output dynamic range is available. Output input impedance of the driven circuit is 100 times ROUT, dynamic range is limited by both the maximum allowed then the accuracy of VOUT will be reduced by 1% since: output current and the maximum allowed output voltage, R as well as the minimum practical output signal. If less V OUT • RIN(DRIVEN) OUT = IOUT • dynamic range is required, then R R IN can be increased OUT +RIN(DRIVEN) accordingly, reducing the maximum output current and power dissipation. If low sense currents must be resolved =IOUT •ROUT • 100 = 0.99 •IOUT •ROUT accurately in a system that has a very wide dynamic range, 101 a smaller RIN than the maximum current spec allows may
Error Sources
be used if the maximum current is limited in another way, such as with a Schottky diode across R The current sense system uses an amplifier and resistors SENSE (Figure 3). This will reduce the high current measurement accuracy to apply gain and level shift the result. The output is then by limiting the result, while increasing the low current dependent on the characteristics of the amplifier, such as measurement resolution. gain and input offset, as well as resistor matching. Ideally, the circuit output is: V+ V R OUT = VSENSE • ROUT ; VSENSE = RSENSE • ISENSE SENSE DSENSE RIN 6106 F03 LOAD In this case, the only error is due to resistor mismatch, which provides an error in gain only. However, offset volt-
Figure 3. Shunt Diode Limits Maximum Input Voltage to Allow Better Low Input Resolution Without Overranging
age and bias current cause additional errors. 6106fb 8 For more information www.linear.com/LT6106