Datasheet LTC6104 (Analog Devices) - 9

LTC6104
APPLICATIONS INFORMATION Selection of External Input Resistor, RIN
–IN terminals will increase the effective RIN value, causing a gain error, especially for small R The external input resistor, R IN values. In addition, IN, controls the trans- internal device resistance will add approximately 0.3Ω conductance of the current sense circuit. to RIN. V 1 Since I = SENSE , transconductance g Trace and interconnect impedances to the +INB terminal will OUT m = RIN RIN have an effect on offset error. These errors are described in more detail later in this data sheet. V For example if R = 100 , Ω then I S , E ENSE IN OUT = or 100Ω
Selection of External Output Resistor, ROUT
I = ± m 1 A for V = ± m 100 V. OUT SENSE The output resistor, ROUT, determines how the output cur- rent is converted to voltage. VOUT is simply IOUT • ROUT + RIN should be chosen to allow the required resolution VREF. In choosing an output resistor, the maximum output while limiting the output current. At low supply voltage, voltage range must fi rst be considered. If the circuit that IOUT may be as much as ±1mA. By setting RIN such that is driven by the output does not limit the output voltage the largest expected sense voltage gives IOUT = ±1mA, range, then ROUT must be chosen such that the maximum then the maximum output dynamic range is available. output voltage range does not exceed the LTC6104 maxi- Output dynamic range is limited by both the maximum mum output voltage range (see Electrical Characteristics). allowed output current and the maximum allowed output If the following circuit is a buffer or ADC with limited input voltage, as well as the minimum practical output signal. If range, then ROUT must be chosen so that VOUT is in the less dynamic range is required, then RIN can be increased allowed maximum input range of this circuit. accordingly, reducing the maximum output current and power dissipation. If low sense currents must be resolved In addition, the output impedance is determined by ROUT. accurately in a system that has very wide dynamic range, If the circuit to be driven has high enough input imped- a smaller R ance, then almost any useful output impedance will be IN than the maximum current specifi cation allows may be used if the maximum current is limited in acceptable. However, if the driven circuit has relatively low another way, such as with a Schottky diode across R input impedance, or draws spikes of current, such as an SENSE (Figure 3). This will reduce the high current measurement ADC might do, then a lower ROUT value may be required accuracy by limiting the result, while increasing the low in order to preserve the accuracy of the output. As an current measurement resolution. This approach can be example, if the input impedance of the driven circuit is helpful in cases where occasional large burst currents 100 times ROUT, then the accuracy of VOUT will be reduced may be ignored. by 1% since: Care should be taken when designing the printed circuit R • R OUT IN DRIV ( EN) V – V = I • board layout to minimize input trace resistance (to Pins OUT REF OUT R + R OUT IN DRI ( V VEN) 5, 6, 7 and 8). Trace and interconnect impedances to the = 100 OUT I • ROUT • = . 0 99 • OUT I • OUT R RSENSE 101 LOAD BATTERY
Selection of External Voltage Reference, VREF
DSENSE Selection of external reference voltage should be consid- 6104 F03 ered together with selection of ROUT.
Example: Figure 3. Shunt Diodes Limit Maximum Input Voltage to Allow Better Low Input Resolution Without Overranging
Given the conditions: IOUT = –1mA to 1mA, VS = 12V. 6104f 9