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SA5230 Datasheet(PDF) 10 Page - NXP Semiconductors |
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SA5230 Datasheet(HTML) 10 Page - NXP Semiconductors |
10 / 17 page Philips Semiconductors Product specification NE/SA5230 Low voltage operational amplifier 1994 Aug 31 10 The NE5230 circuit shown in Figure 6 is a pseudo transistor configuration. The inverting input is equivalent to the “base,” the point where VEE and the non-inverting input meet is the “emitter,” and the connection after the output diode meets the VCC pin is the collector. The output diode is essential to keep the output from saturating in this configuration. From here it can be seen that the base and emitter form a voltage-follower and the voltage present at RC must equal the input voltage present at the inverting input. Also, the emitter and collector form a current-follower and the current flowing through RC is equivalent to the current through RL and the amplifier. This sets up the current loop. Therefore, the following equation can be formulated for the working current transmission line. The load current is: IL=VIN/RC (2) and proportional to the input voltage for a set RC. Also, the current is constant no matter what load resistance is used while within the operating bandwidth range of the op amp. When the NE5230’s supply voltage falls past a certain point, the current cannot remain constant. This is the “voltage compliance” and is very good for this application because of the near rail output voltage. The equation that determines the voltage compliance as well as the largest possible load resistor for the NE5230 is as follows: RL max=[Vremote supply)-VCC min- VIN max]/IL (3) Where VCC min is the worst-case power supply voltage (approximately 1.8V) that will still keep the part operational. As an example, when using a 15V remote power supply, a current sensing resistor of 1 Ω, and an input voltage (VIN) of 20mV, the output current (IL) is 20mA. Furthermore, a load resistance of zero to approximately 650 Ω can be inserted in the loop without any change in current when the bias current-control pin is tied to the negative supply pin. The voltage drop across the load and line resistance will not affect the NE5230 because it will operate down to 1.8V. With a 15V remote supply, the voltage available at the amplifier is still enough to power it with the maximum 20mA output into the 650 Ω load. What this means is that several instruments, such as a chart recorder, a meter, or a controller, as well as a long cable, can be connected in series on the loop and still obtain accurate readings if the total resistance does not exceed 650 Ω. Furthermore, any variation of resistance in this range will not change the output current. Any voltage output type transducer can be used, but one that does not need external DC voltage or current excitation to limit the maximum possible load resistance is preferable. Even this problem can be surmounted if the supply power needed by the transducer is compatible with the NE5230. The power goes up the line to the transducer and amplifier while the transducer signal is sent back via the current output of the NE5230 transconductance configuration. The voltage range on the input can be changed for transducers that produce a large output by simply increasing the current sense resistor to get the corresponding 4 to 20mA output current. If a very long line is used which causes high line resistance, a current repeater could be inserted into the line. The same configuration of Figure 6 can be used with exception of a resistor across the input and line ground to convert the current back to voltage. Again, the current sensing resistor will set up the transconductance and the part will receive power from the line. TEMPERATURE TRANSDUCER A variation on the previous circuit makes use of the supply current control pin. The voltage present at this pin is proportional to absolute temperature (PTAT) because it is produced by the amplifier bias current through an internal resistor divider in a PTAT cell. If the control pin is connected to the input pin, the NE5230 itself can be used as a temperature transducer. If the center tap of a resistive pot is connected to the control pin with one side to ground and the other to the inverting input, the voltage can be changed to give different temperature versus output current conditions (see Figure 7). For additional control, the output current is still proportional to the input voltage differential divided by the current sense resistor. When using the NE5230 as a temperature transducer, the thermal considerations in the previous section must be kept in mind. V REMOTE POWER SUPPLY NE5230 VCC VEE IOUT 3 2 4 5 6 7 + – RC RL + – NOTES: 1. IOUT = VIN/RC 2. RL MAX ≈ V REMOTE * 1.8V * V INMAX I OUT For RC = 1Ω I OUT 4mA 20mA V IN 4mV 20mV 10 200 SL00256 Figure 7. NE5230 Remote Temperature Transducer Utilizing 4-20mA Current Transmission. This Application Shows the use of the Accessibility of the PTAT Cell in the Device to Make the Part, Itself, a Transducer |
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