Features: High cmv isolation: 2500v rms continuous; 63500 V peak continuous; small size: 1.00" 2.10" 0.350" 33; three-port isolation: input, output and power; low nonlinearity: 0.012% max6; wideband: 20kHz full power (–3dB); Low Gain Drift: 25 ppm/c68 max; High CMR: 120dB (G=100V/V); Isolation Power: 15V@5mA Uncommitted Input Amplifier 66.

Applications: Multiplex data acquisition; high voltage instrumentation amplifiers; current shunt measurements; process signal isolation.

General Instructions

The AD210 is the latest addition to a new generation of low-cost, high-performance isolation amplifiers. This three-port, broadband isolation amplifier is fabricated from surface mount components in an automated assembly process. The AD210 combines design expertise with state-of-the-art manufacturing techniques to produce an extremely compact and economical isolator whose performance and rich user features far exceed more expensive devices.

The AD210 provides a complete isolation function, providing signal and power isolation through transformer coupling inside the module. The fully functional design of the AD210, powered by a +15V supply, eliminates the need for an external DC/DC converter, unlike optocoupled isolation devices. A true three-port design allows the AD210 to be used as an input or output isolator in single- or multi-channel applications. Under sustained common mode stress, the AD210 will maintain its high performance.

Offering high accuracy and complete galvanic isolation, the AD210 interrupts ground loops and leakage paths, and rejects common-mode voltages and noise that can degrade the measurement accuracy of other vices. In addition, the AD210 provides protection against fault conditions that could damage other parts of the measurement system.

Product Highlights

The AD210 is a full-featured isolator that offers many benefits to the user, including: High common-mode performance: The AD210 provides 2500 V rms (continuous) and ±3500 V peak (continuous) common - between any two ports mode voltage isolation. A low input capacitance of 5 pf yields 120 db cmr at a gain of 100 with low leakage current (2µA rms max at 240 v rms, 60 Hz).

High precision: maximum nonlinearity is ±0.012% (class b), maximum gain drift is ±25 ppm/°C, input offset drift is (±10±30/g) μV/°C, AD210 provides high-level isolation while ensuring signal integrity.

Broadband: The full power bandwidth of the AD210 is 20kHz, which makes it useful for wideband signals. It is also effective in applications such as control loops, where limited bandwidth can lead to instability.

Small Form Factor: The AD210 provides complete isolation in a small DIP package of only 1.00" x 2.10" x 0.350". The low profile DIP package allows for 0.5" card frames and assemblies. The leads are optimized for easy board layout while maintaining isolation spacing between ports.

Three-Port Design: The AD210's three-port design structure allows each port (input, output, and power) to remain independent. This three-port design allows the AD210 to be used as an input or output isolator. It also provides additional system protection in the event of a power failure.

Isolated Power Supply: Provides ±15 V@5 mA on the input and output sections of the isolator. This feature allows the AD210 to excite floating signal conditioners, front-end amplifiers, and remote sensors, among other circuits, at the input and output.

Flexible Input: Provides an uncommitted op amp at the input. The amplifier provides buffering and gain as needed and facilitates many alternative input functions depending on the user's needs.

Inside AD210

The basic block diagram of AD210 is shown in Figure 1. The +15V power supply is connected to the power port and provides ±15V isolated power to the input and output ports through a 50kHz carrier frequency. Uncommitted input amplifiers can be used to provide gain or buffering of the input signal to the AD210. The full wave modulator converts the signal to the carrier frequency to be applied to the transformer t1. The synchronous demodulator at the output port reconstructs the input signal. To reduce output noise and ripple, a 20khz three-pole filter is used. Finally, the output buffer provides a low impedance output capable of driving a 2kΩ load.

Using AD210

AD210 is very simple and can be widely used. The AD210 is powered by a +15V supply and will provide excellent performance when used as an input or output isolator in single-channel and multi-channel configurations.

Input Configuration: Figure 2 shows the basic unity-gain configuration for signals up to ±10 V. Changes to other input amplifiers are shown below. For smaller signal levels, Figure 3 shows how gain can be achieved while maintaining high input impedance.

Figure 3. Basic Unity Gain Configuration

The high input impedance 3 of the circuit in Figure 3 can be maintained in inversion applications. Since the AD210 is a three-port isolator, the input leads or output leads can be interchanged to produce signal inversion.

Figure 4 shows how to accommodate a current input or the sum of current or voltage. This circuit configuration can also be used for signals greater than ±10 V. For example, an input range of ±100 V can be handled with rf=20 kΩ and rs1=200 kΩ.

Adjustment

When gain and offset adjustment is required, the actual circuit adjustment components will depend on the choice of input configuration and whether the adjustment is made at the input or output of the isolator. Adjustment on the output side can be used when the potentiometer on the input side represents a hazard due to the presence of high common-mode voltages during adjustment. Offset adjustment is best done at the input as it is best to zero the offset before gain.

Figure 5 shows the input conditioning circuit used when the input amplifier is configured in non-converting mode. This offset adjustment circuit passes through the low side of the signal source. This will not work if the power supply has another current path into the common or if current is flowing in the signal source lo leads. To minimize CMR degradation, keep resistors in series with input lo less than a few hundred ohms.

Figure 5 also shows the preferred gain adjustment circuit. The circuit shows an RF of 50 kΩ and will operate with a gain of 10 or greater. At lower gains, this adjustment becomes less effective (its effect is halved at g=2), so at lower gains pot must be a larger fraction of the total rf. At g=1 (follower), the gain cannot be adjusted down without affecting the input impedance; it is best to adjust the gain after the source or output.

Figure 6 shows the input conditioning circuit used when the input amplifier is configured in inversion mode. The offset adjustment makes the voltage at the summing node zero. This is preferable to current injection because it is less affected by subsequent gain adjustments. Gain adjustments are made in feedback and will be used for gains from 1 V/V to 100 V/V.

Figure 7 shows how the offset adjustment is done at the output through the offset floating output port. In this circuit, the ±15 V will be powered by a separate supply. The output amplifier of the AD210 is fixed at one unit, therefore, the output gain must be in subsequent stages.

PCB layout for multi-channel applications: Unique pin positioning minimizes board space constraints for multi-channel applications. Figure 8 shows the recommended printed circuit board layout for a non-vertical input configuration with gain.

Synchronization: The AD210 is insensitive to the clocks of adjacent units, eliminating the need for synchronizing clocks. However, in rare cases, if the input signal lines are tied together, channel-to-channel pickup can occur. If this happens, a shielded input cable is recommended.

Performance characteristics

Common-Mode Rejection: Figure 9 shows the AD210's common-mode rejection over frequency, gain, and input source resistance. To maximize rejection of unwanted signal common mode, keep the input source resistance low, and route the input carefully to avoid excessive stray capacitance at the input terminals.

Phase Shift: Figure 10 illustrates the AD210's low phase shift and gain versus frequency. The phase shifting and broadband performance of the AD210 make it ideal for applications such as power monitoring systems.

Input Noise vs Frequency: Voltage noise at the input depends on gain and signal bandwidth. Figure 11 illustrates the typical input noise of the AD210 in nv/√Hz over the 10 to 10 kHz frequency range.

Gain Nonlinearity vs Output: Gain nonlinearity is defined as the deviation of the output voltage from the best straight line and is specified as a peak-to-peak percentage of the output span. The AD210B has a guaranteed maximum nonlinearity of ±0.012% and an output range of ±10V. The nonlinear performance of the AD210 is shown in Figure 12.

Gain Nonlinearity vs. Output Swing: The gain nonlinearity of the AD210 varies with the total signal swing. Gain nonlinearity is improved as part of the signal swing when the output swing is less than 20v. The shape of the nonlinearity remains the same. Figure 13 shows the gain nonlinearity of the AD210 as a function of total signal swing.

Gain vs. Temperature: Figure 14 shows the gain vs. temperature performance of the AD210. The gain versus temperature performance shown is for the ad210 configured as a unity gain amplifier.

Isolated Power: The AD210 provides isolated power at the input and output ports. This power supply can be used for a variety of signal conditioning tasks. Both ports are rated for a nominal ±15 V5 mA.

The load characteristics of the isolated power supply are shown in Figure 15. For example, when measuring load shedding of an input isolated power supply viss, the load is placed between +viss and –viss. The curves labeled viss and vos are the individual load shedding characteristics of the input and output power supplies, respectively.

There is also some effect on one of the isolated power supplies when the other power supply is loaded. The curve labeled Cross Load represents the sensitivity of the input or output supply as a function of reverse supply load.

Finally, the curves labeled voss-simultaneous and viss-simultaneous show the load characteristics of an isolated power supply when the two power supplies are equally loaded.

The AD210 provides short-circuit protection for its isolated power supply. When the input power supply or the output power supply is short-circuited to the input common line or the output common line, respectively, no damage will be caused even in the case of a continuous short circuit. However, if the input and output supplies are shorted at the same time, the AD210 can be damaged.

In any case, care should be taken to ensure that the power supply is not accidentally shorted. The ripple of an isolated power supply varies with the load. Figure 16a shows this relationship. The AD210 has internal bypass capacitors to reduce the ripple to a level that does not affect performance even at full load. Since the internal circuitry is more sensitive to noise on negative supplies, these supplies are filtered more heavily. If a particular application requires more bypassing of the isolated power supply, then adding external capacitors is no problem. Figure 16b depicts power supply ripple at full load as a function of external bypass capacitors.

Applications

Noise reduction in data acquisition systems: Transformer-coupled isolation amplifiers must have a carrier to pass both AC and DC signals through their signal transformers. Therefore, some carrier ripple is inevitably passed to the isolator output. As the bandwidth of the isolator increases, more carrier signal appears at the output. In most cases, the ripple at the output of the ad210 will be insignificant compared to the measured signal. However, in some applications, especially when using a fast analog-to-digital converter after an isolator, it may be necessary to add filtering; otherwise the ripple may cause inaccurate measurements. The circuit shown in Figure 17 will limit the bandwidth of the isolator, thereby reducing carrier ripple.

Self-powered current source

The output circuit shown in Figure 18 can be used to create a self-powered output current source using the ad210. A 2 kΩ resistor converts the voltage output of the AD210 to an equivalent current vout/2 kΩ. This resistor directly affects the output gain temperature coefficient and must have suitable application stability. External low power op amp with +VOSS and –VOSS maintaining its summing junction at output common. All current flowing through the 2 kΩ resistor flows through the output Darlington pass device. The Darlington structure is used to minimize the loss of output current to the base.

A low leakage diode is used to protect the base emitter junction from reverse bias. Using –VOSS as the current return allows compliance over 10 V. Offset and gain control can be done at the input of the AD210, or by changing the 2kΩ resistor and summing the small correction current directly to the summing node. A nominal range of 1-5 mA is recommended because the current output cannot reach zero due to reverse bias and leakage current. If the AD210 is powered by the input potential, the circuit provides a fully isolated wide bandwidth current output. This configuration is limited to 5mA output current.

Isolated v-to-i converter

As shown in Figure 19, the AD210 is used to convert a 0 V to +10 V input signal to an isolated 4–20 mA output current. The AD210 isolates input signals from 0 V to +10 V and provides a proportional voltage at the output of the isolator. The output circuit converts the input voltage to an output current of 4–20 mA, which is then applied to the loop load load.

Isolated Thermocouple Amplifier

The AD210 application shown in Figure 20 provides amplification, isolation, and cold junction compensation for a standard J-type thermocouple. The AD590 temperature sensor accurately monitors the input terminal (cold junction). Changes in ambient temperature sensed by the AD590 from 0°C to +40°C cancel at the cold junction. The total circuit gains are 183; 100 and 1.83, respectively, from a1 and ad210. Replace the thermocouple junction with a normal thermocouple wire and a millivolt supply set to 0.0000 V (0 °C) and adjust the output voltage Ro to 0.000 V. Set the millivolt supply to +0.02185 V (400°C) and adjust the output voltage Rg to +4.000 V. This application circuit will produce a nonlinear output of about +10 mV/°C over the range of 0°C to +400°C.

Precision Floating Programmable Reference When the AD210 is combined with a digital-to-analog converter, it can be used to generate a fully floating voltage output. Figure 21 shows a possible implementation. The digital inputs of the AD7541 are TTL or CMOS compatible. Both the AD7541 and AD581 voltage references are powered by the isolated power supply +VISS. ICOM should be connected to the input digital common to provide a digital ground reference for the input.

The AD7541 is a current output DAC, so an external output amplifier is required. The uncommitted input amplifier inside the AD210 can be used for this purpose. For best results, its input offset voltage must be adjusted as shown.

The output voltage of the AD210 will vary from 0 V to –10 V for 0 and full-scale digital inputs, respectively. However, since the output ports are truly isolated, vout and ocom can be freely swapped for voltages from 0v to +10v. The circuit provides a programmable reference voltage with an accuracy of 0 V–10 V and a common-mode range of ±3500 V.

Figure 22 shows a four-channel data acquisition front end used to condition and isolate several common input signals found in various process applications. In this application, each ad210 will provide complete isolation from input to output and from channel to channel. By using one isolator per channel, maximum protection and rejection of unwanted signals can be obtained. The three-port design allows the AD210 to be configured as an input or output isolator. In this application, the isolator is configured as an input device, and the power port provides additional protection against possible power failure.

Channel 1: AD210 is used to convert a 4–20 mA current loop input signal to a 0 V–10 V input. A 25Ω shunt resistor converts 4-20mA of current into a +100mV to +500mV signal. The signal was offset by –100 mV by reverse osmosis to produce an input from 0 mV to +400 mV. This signal is amplified by a gain of 25 to produce the desired 0v to +10v output. If left open, the AD210 will show -2.5 V at the output.

Channel 2: In this channel, the ad210 is used to condition and isolate the ad590 type current output temperature sensor. At +25°C, the AD590 draws a nominal current of 298.2 µA. This current level will change at a rate of 1 microamp/degree Celsius. At -17.8°C (0°F), the AD590 current will decrease by 42.8µA to +255.4µA. The AD580 reference circuit supplies equal but opposite currents, resulting in zero net current, resulting in a 0V output from the AD210. At +100°C (+212°F), the AD590 current output will be 373.2µA minus the 255.4µA bias current from the AD580 circuit to produce an input current of +117.8µA. This current is converted to voltage by rf and rg to produce an output of +2.12v. Channel 2 will produce an output of +10mv/°f from 0°f to +212°f.

Channel 3: Channel 3 is a low level input channel equipped with a high gain amplifier for conditioning millivolt signals. and

With the AD210's input set to unity and the input amplifier set to 1000, A ±10 mV input will produce ±10 V at the output of the AD210.

Channel 4: Channel 4 illustrates one possible configuration of the conditioning bridge circuit. AD584 generates +10V excitation voltage, and A1 inverts the voltage, resulting in negative excitation. A2 provides a gain of 1000 V/V to amplify low-level bridge signals. Additional gain can be obtained by reconfiguring the AD210's input amplifier. ? VISS provides the complete power supply for this circuit, eliminating the need for a separate isolated excitation source.

Each channel is individually addressed by the multiplexer's channel selection. Additional filtering or signal conditioning should be done after the multiplexer before the analog-to-digital conversion stage.