Datasheet LTC6104 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | High Voltage, High Side, Bi-Directional Current Sense Amplifier |
| Pages / Page | 16 / 10 — APPLICATIONS INFORMATION. Output Error, EOUT, Due to the Amplifi er DC … |
| File Format / Size | PDF / 317 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Output Error, EOUT, Due to the Amplifi er DC Offset. Voltage, VOS. Output Error, E
Model Line for this Datasheet
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LTC6104
APPLICATIONS INFORMATION
From the Electrical Characteristics of the LTC6104, the
Output Error, EOUT, Due to the Amplifi er DC Offset
output voltage range is 0.3V to 8V.
Voltage, VOS
If the circuit that is driven by the output limits the maxi- R E = V OUT • mum output voltage to ≈5V, to achieve maximum dynamic OUT(VOS) OS RIN range, V OUT should be 0.3V for –1mA IOUT and 5V for 1mA IOUT. The DC offset voltage of the amplifi er adds directly to the 5V – 0.3V value of the sense voltage, VSENSE. This is the dominant R = = 2.3 k OUT 5 , error of the system and it limits the available dynamic mA 2 range. The section, Selection of External Current Sense 5 – 0.3 Resistor, provides details. V = 0.3 + = 2 6 . 5V REF 2
Output Error, E + OUT, Due to the Bias Currents, IB and –
A standard 2.5V reference could be used in this example.
IB
With I + OUT = ±1mA and ROUT = 2.2k, the output voltage The bias current IB fl ows into the positive input of the range would equal 0.3V to 4.7V internal op amp. I – B fl ows into the negative input.
V
⎛ R ⎞
REF Considerations
E = R I + SENSE • – I – OUT IB ( IAS) OUT B ⎝⎜ R B V IN ⎠⎟ REF as shown in Figure 1, provides a positive offset so that the output can swing above and below this point. It is Since I + ≈ I – = I recommended that this is an accurate voltage reference. B B BIAS, if RSENSE << RIN then: Most voltage references will work in this application as EOUT(IBIAS) ≈ –ROUT • IBIAS long as they are able to sink and source current. Make sure For instance if I that the device maintains the required voltage accuracy as BIAS is 100nA and ROUT is 1k, the output error is 0.1mV. the current varies through its entire range.
Output Error, E Error Sources OUT, Due to the Finite DC Open-Loop Gain, AOL, of the LTC6104 Amplifi er
The current sense system uses an amplifi er and resistors This error is inconsequential as the A to apply gain and level shift the result. The output is then OL of the LTC6104 is very large. dependent on the characteristics of the amplifi er, such as gain and input offset, as well as resistor matching. Ideally,
Example:
the circuit output is: V V If an I OUT = 3 R SENSE range = (±1mA to ±1A) and I A 1 V – V = I • R = V OUT SENSE OUT REF OUT OUT SENSE • , RIN Then, from the Electrical Characteristics of the V = R • I LTC6104: SENSE S SENSE SENSE V In this case, the only error is due to resistor mismatch, SENSE M ( AX) mV 500 R ≈ = = m SENSE 500 Ω which provides an error in gain only. However, offset I A SENSE M ( AX) 1 voltage, bias current and fi nite gain in the amplifi er cause additional errors. R VOUT MA ( X) 3V G Gain OUT = = = = 6 R V mV 500 IN SENSE M ( AX) 6104f 10
APPLICATIONS INFORMATION
From the Electrical Characteristics of the LTC6104, the
Output Error, EOUT, Due to the Amplifi er DC Offset
output voltage range is 0.3V to 8V.
Voltage, VOS
If the circuit that is driven by the output limits the maxi- R E = V OUT • mum output voltage to ≈5V, to achieve maximum dynamic OUT(VOS) OS RIN range, V OUT should be 0.3V for –1mA IOUT and 5V for 1mA IOUT. The DC offset voltage of the amplifi er adds directly to the 5V – 0.3V value of the sense voltage, VSENSE. This is the dominant R = = 2.3 k OUT 5 , error of the system and it limits the available dynamic mA 2 range. The section, Selection of External Current Sense 5 – 0.3 Resistor, provides details. V = 0.3 + = 2 6 . 5V REF 2
Output Error, E + OUT, Due to the Bias Currents, IB and –
A standard 2.5V reference could be used in this example.
IB
With I + OUT = ±1mA and ROUT = 2.2k, the output voltage The bias current IB fl ows into the positive input of the range would equal 0.3V to 4.7V internal op amp. I – B fl ows into the negative input.
V
⎛ R ⎞
REF Considerations
E = R I + SENSE • – I – OUT IB ( IAS) OUT B ⎝⎜ R B V IN ⎠⎟ REF as shown in Figure 1, provides a positive offset so that the output can swing above and below this point. It is Since I + ≈ I – = I recommended that this is an accurate voltage reference. B B BIAS, if RSENSE << RIN then: Most voltage references will work in this application as EOUT(IBIAS) ≈ –ROUT • IBIAS long as they are able to sink and source current. Make sure For instance if I that the device maintains the required voltage accuracy as BIAS is 100nA and ROUT is 1k, the output error is 0.1mV. the current varies through its entire range.
Output Error, E Error Sources OUT, Due to the Finite DC Open-Loop Gain, AOL, of the LTC6104 Amplifi er
The current sense system uses an amplifi er and resistors This error is inconsequential as the A to apply gain and level shift the result. The output is then OL of the LTC6104 is very large. dependent on the characteristics of the amplifi er, such as gain and input offset, as well as resistor matching. Ideally,
Example:
the circuit output is: V V If an I OUT = 3 R SENSE range = (±1mA to ±1A) and I A 1 V – V = I • R = V OUT SENSE OUT REF OUT OUT SENSE • , RIN Then, from the Electrical Characteristics of the V = R • I LTC6104: SENSE S SENSE SENSE V In this case, the only error is due to resistor mismatch, SENSE M ( AX) mV 500 R ≈ = = m SENSE 500 Ω which provides an error in gain only. However, offset I A SENSE M ( AX) 1 voltage, bias current and fi nite gain in the amplifi er cause additional errors. R VOUT MA ( X) 3V G Gain OUT = = = = 6 R V mV 500 IN SENSE M ( AX) 6104f 10
