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9 page 
LTC6104 9 6104f APPLICATIONS INFORMATION Selection of External Input Resistor, RIN The external input resistor, RIN, controls the trans- conductance of the current sense circuit. Since I transconductance g OUT m == V R SENSE IN , 1 R R For example if R then I V IN IN OUT S ,, == 100 Ω E ENSE OUT SENSE or I mA for V mV 100 1 100 Ω . =± =± RIN should be chosen to allow the required resolution while limiting the output current. At low supply voltage, IOUT may be as much as ±1mA. By setting RIN such that the largest expected sense voltage gives IOUT = ±1mA, then the maximum output dynamic range is available. Output dynamic range is limited by both the maximum allowed output current and the maximum allowed output voltage, as well as the minimum practical output signal. If less dynamic range is required, then RIN can be increased accordingly, reducing the maximum output current and power dissipation. If low sense currents must be resolved accurately in a system that has very wide dynamic range, a smaller RIN than the maximum current specification allows may be used if the maximum current is limited in another way, such as with a Schottky diode across RSENSE (Figure 3). This will reduce the high current measurement accuracy by limiting the result, while increasing the low current measurement resolution. This approach can be helpful in cases where occasional large burst currents may be ignored. Care should be taken when designing the printed circuit board layout to minimize input trace resistance (to Pins 5, 6, 7 and 8). Trace and interconnect impedances to the –IN terminals will increase the effective RIN value, causing a gain error, especially for small RIN values. In addition, internal device resistance will add approximately 0.3 Ω to RIN. Trace and interconnect impedances to the +INB terminal will have an effect on offset error. These errors are described in more detail later in this data sheet. Selection of External Output Resistor, ROUT The output resistor, ROUT, determines how the output cur- rent is converted to voltage. VOUT is simply IOUT • ROUT + VREF. In choosing an output resistor, the maximum output voltage range must first be considered. If the circuit that is driven by the output does not limit the output voltage range, then ROUT must be chosen such that the maximum output voltage range does not exceed the LTC6104 maxi- mum output voltage range (see Electrical Characteristics). If the following circuit is a buffer or ADC with limited input range, then ROUT must be chosen so that VOUT is in the allowed maximum input range of this circuit. In addition, the output impedance is determined by ROUT. If the circuit to be driven has high enough input imped- ance, then almost any useful output impedance will be acceptable. However, if the driven circuit has relatively low input impedance, or draws spikes of current, such as an ADC might do, then a lower ROUT value may be required in order to preserve the accuracy of the output. As an example, if the input impedance of the driven circuit is 100 times ROUT, then the accuracy of VOUT will be reduced by 1% since: VV I RR RR OUT REF OUT OUT IN DRIVEN OUT IN DRI –• • () ( = + V VEN OUT OUT OUT OUT IR IR ) •• . • • == 100 101 099 Selection of External Voltage Reference, VREF Selection of external reference voltage should be consid- ered together with selection of ROUT. Example: Given the conditions: IOUT = –1mA to 1mA, VS = 12V. Figure 3. Shunt Diodes Limit Maximum Input Voltage to Allow Better Low Input Resolution Without Overranging DSENSE RSENSE BATTERY 6104 F03 LOAD |
