The HCPL-7800 series of isolation amplifiers are designed for current sensing in electronic motor drives. In typical automotive and industrial applications, the HCPL-7800 detects the analog voltage drop caused by motor current passing through an external resistor. The HCPL-7800 isolation amplifier is based on Avago's optocoupler and optoelectronic technology.

Figure 3 shows the main functional blocks of the HCPL-7800 in operation, the sigma-delta analog-to-digital converter converts the analog input signal into a high-speed serial bit stream, time-averaged, which is directly proportional to the input signal. This high-speed digital data stream is encoded and optically transmitted to the detection circuit. The detected signal is decoded and converted to an accurate analog voltage level, which is then filtered to produce the final output signal. To help maintain device accuracy over time and temperature the internal amplifiers are chopper stable. In addition, the encoder circuit eliminates the effect of pulse width distortion by optically transmitting data by generating a pulse for each edge (rising and falling edge) that the converter data is propagated through, essentially converting the width of the sigma-delta output The output pulse of the encoder that pulses into the position. A significant benefit of this coding scheme is that any non-ideal LED characteristics (such as nonlinearity and drift over time and temperature) have little, if any, effect on the performance of the HCPL-7800.

circuit information

The recommended application circuit is shown in Figure 1. The floating supply (which in many applications may be, is used to drive the same supply as the high-side power transistor) is regulated to 5V with a simple three-terminal voltage regulator. The input of the HCPL-7800 is connected directly to the current sensing resistor. Differentiating the output of this isolation amplifier is converted to a ground referenced single-ended output voltage with a simple differential amplifier circuit. Although the application circuit is relatively simple, a few general recommendations should be followed to ensure optimum performance. As shown in Figure 1 0.1µF bypass capacitors should be located as close as possible to the input and output power pins of the HCPL-7800. Notice that pin 2 (VIN+) is bypassed with a 0.01µF capacitor to reduce the input bias voltage, which can be caused by the combination of long input leads and the capacitor nature of the switched input circuit. In parallel with pin 3 (VIN-) directly to pin 4 (GND1), this power return line also serves as the negative side current sense resistor of the sense pair; this allows a single twisted pair wire to connect the isolation amplifier to sense the resistor. In some applications, however, better performance can be obtained by connecting pins 2- and (VIN+ and VIN-) directly through the sense resistor with the twisted pair and using separate lines for the power supply return line. Both input pins should be bypassed with 0.01µF capacitors close to the isolation amplifier. In either case, it is recommended that a twisted-pair of wires be used to connect the isolation amplifier to the current sense resistor to minimize EMI to the sensed signal. For best CMR perfor- mance, the printed layout circuit board (PCB) should minimize any parasitic coupling maintaining the maximum distance possible between the input and output side circuits, and ensure that any ground planes on the PCB are not An example single-sided application circuit directly through the HCPL-7800 below for the recommended PCB layout is shown in Figure 4. The trace mode is shown in Figure 4 as it would appear in a "see-through" view viewed from above the PCB; a mirror image of this layout can be is used to generate a PCB. The inexpensive 78L05 three-terminal regulator is shown in the recommended application circuit. Since the performance of the isolation amplifier can be affected by changing the supply voltage, using a tighter output voltage tolerance with regulation will result in better overall circuit performance. There are many different voltage regulators that provide tighter output voltage tolerance than the 78L05 can be used, including: TL780-05 (Texas Instruments), LM340LAZ-5.0 and LP2950CZ-5.0 (National Semiconductor). Post-amplifier circuits used externally should be sufficiently accurate that it does not contribute a significant amount of offset or offset drift relative to the contribution from the isolated amplifier. In general, op amps exhibit better performance-to-deviation ratios with bipolar input stages than op amps with JFET or MOSFET input stages. In addition, the op amp should also have sufficient bandwidth and slew rate so that it does not reflect the overall circuit configuration that adversely affects speed. The post-amplifier circuit consists of a pair of capacitors (C5 and C6) in the form of a unipolar low-pass filter; these capacitors allow the bandwidth of the post-amp to be adjusted independently of the gain and are useful in order to reduce noise from the isolated output of the amplifier. There are many different op amps that can be used in the circuit, including: MC34082A, TL032A, TLO52A and TLC277 (Texas Instruments), LF412A (National Semiconductor). The gain setting resistors of the post amplifier should have a tolerance of 1% or better to ensure adequate CMRR and adequate gain tolerance for the overall circuit. A resistor network can be used with better specific tolerance than can be achieved using discrete resistors. Resistor networks can also reduce the number of components in the total circuit and the required board space.

The current sense resistor should have a relatively low value resistance to minimize power dissipation, a fairly low inductance to accurately reflect high frequency signal compensation, and a reasonably tight tolerance to overall maintain circuit accuracy. While reducing the sense of value of the resistor reduces power dissipation, it also reduces the full-scale input voltage decision more significantly than the effect of amplifier offset voltage. These two conflicting considerations, therefore, must be weighed against each other in choosing the appropriate sense resistor for the specific application. To maintain circuit accuracy, it is recommended that the sense resistor and isolation amplifier circuit be as close to each other as possible. While it is also possible to buy current-sensing resistors from established suppliers (for example, LVR-1, -3 and -5 resistors from Valley), it is also possible to make sense resistors with a small piece of wire or even trace amounts on a PC board superior.