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10 page 
LTC6104 10 6104f APPLICATIONS INFORMATION From the Electrical Characteristics of the LTC6104, the output voltage range is 0.3V to 8V. If the circuit that is driven by the output limits the maxi- mum output voltage to ≈5V, to achieve maximum dynamic range, VOUT should be 0.3V for –1mA IOUT and 5V for 1mA IOUT. R VV mA k VV OUT REF == =+ = 50 3 2 235 03 50 3 2 265 –. ., . –. . A standard 2.5V reference could be used in this example. With IOUT = ±1mA and ROUT = 2.2k, the output voltage range would equal 0.3V to 4.7V VREF Considerations VREF as shown in Figure 1, provides a positive offset so that the output can swing above and below this point. It is recommended that this is an accurate voltage reference. Most voltage references will work in this application as long as they are able to sink and source current. Make sure that the device maintains the required voltage accuracy as the current varies through its entire range. Error Sources The current sense system uses an amplifier and resistors to apply gain and level shift the result. The output is then dependent on the characteristics of the amplifier, such as gain and input offset, as well as resistor matching. Ideally, the circuit output is: VV I R V R R VR OUT REF OUT OUT SENSE OUT IN SENSE –• • , == = SSENSE SENSE I • In this case, the only error is due to resistor mismatch, which provides an error in gain only. However, offset voltage, bias current and finite gain in the amplifier cause additional errors. Output Error, EOUT, Due to the Amplifier DC Offset Voltage, VOS EV R R OUT VOS OS OUT IN () • = The DC offset voltage of the amplifier adds directly to the value of the sense voltage, VSENSE. This is the dominant error of the system and it limits the available dynamic range. The section, Selection of External Current Sense Resistor, provides details. Output Error, EOUT, Due to the Bias Currents, IB + and IB – The bias current IB + flows into the positive input of the internal op amp. IB – flows into the negative input. ER I R R I OUT IBIAS OUT B SENSE IN B () – •– = ⎛ ⎝⎜ ⎞ ⎠⎟ + Since IB + ≈ IB– = IBIAS, if RSENSE << RIN then: EOUT(IBIAS) ≈ –ROUT • IBIAS For instance if IBIAS is 100nA and ROUT is 1k, the output error is 0.1mV. Output Error, EOUT, Due to the Finite DC Open-Loop Gain, AOL, of the LTC6104 Amplifier This error is inconsequential as the AOL of the LTC6104 is very large. Example: If an ISENSE range = (±1mA to ±1A) and V I V A OUT SENSE = 3 1 Then, from the Electrical Characteristics of the LTC6104: R V I mV A m SENSE SENSE MAX SENSE MAX ≈= = () () 500 1 500 Ω G Gain R R V V V mV OUT IN OUT MAX SENSE MAX == = = () () 3 500 6 6 |
