Low-cost high-speed differential amplifier AD8132

Low-cost high-speed differential amplifier AD8132

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Source: carefree e-development network Author: Qu cuixiang Li Gang

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1 Overview
AD8132 is a low-cost amplifier with single-input or differential-mode input and differential-mode output, which can be used to control the gain. AD8132 has its unique internal feedback, which allows you to adjust the output gain. When AD8132 operates at 10 MHz, it can maintain the Phase Equilibrium of-68dB, restrain harmonic waves, and effectively reduce electromagnetic radiation, this is A major improvement in the operational amplifier technology to drive the/D converter of differential mode input or to transmit signals over long distances.
In addition to its low-cost feature, AD8132 is a product based on bipolar XFCB technology. In addition to its low-cost features, AD8132 provides a passthrough frequency of up to 350 MHz and transmission of differential mode signals. It only has the SFDR distortion of-99dBc at 5 MHz. AD8132 does not require A transmitter with A high-performance A/D converter and does not lose low-frequency information or DC information. The common-mode level at the output end of the differential mode can be adjusted by the input voltage of the VOCM pin, in this way, the input level can be quickly translated when A/D converter of A single power supply is driven. The rapid overload recovery capability of AD8132 ensures its sampling accuracy.

AD8132 can also be used as a differential mode signal driver to transmit high-speed signals through a cable or coaxial cable. AD8132 can improve the high-frequency components of the signal by adjusting the feedback coefficient of the feedback network, and drive a twisted cable or coaxial cable consisting of cat3 or cat5, and the line loss is very small. Therefore, it can be widely used in transmission systems that simulate video signals, digital video signals, and other high-speed data.
AD8132 is a differential mode signal operation mode, and can be a reference level in the earth. Therefore, it is not affected by ground noise. It can use a signal chain for differential mode signal processing, such as filtering and amplification, thus greatly simplifying the conversion process between differential mode signal and single input signal.
AD8132 adopts SOIC and μsoic encapsulation. Can work at-40 ℃ ~ + Within the temperature range of 85 ℃.

2Pin Function
Figure 1 shows the AD8132 pin chart, which is listed in Table 1 as its feature description.

3Working Principle
The difference between AD8132 and an ordinary op-amp is that its circuit structure has complementary external input and output. Among them, the input VOCM can be used to control the common mode output voltage, while the output is a single output analog Supplement component of a common op amp. At work, a general op-op usually has only one feedback loop, while AD8132 has two feedback loops. This allows AD8132 to be widely used in various new types of circuits.
A feedback loop of AD8132 can be used to control the common mode output voltage. The VOCM is used as the input signal, and the corresponding output is the average value of the common mode voltage or two differential mode voltages (+ OUT and-OUT. The circuit gain is usually set to unit gain. When AD8132 is working in a linear area, it must comply with the VOUT, cm = VOCM constraints. Another feedback loop determines the differential mode output of the circuit. Similar to a common op amp, the gain of an amplifier can be controlled by negative feedback.

4 AD8132Application Circuit
4.1 Basic Application Circuit

When analyzing the application circuit of AD8132, it should be based on the following assumptions:
● High input impedance, and the load effect can be ignored;
● The input bias current is small enough to be ignored;
● Low output impedance;
● Infinite open-loop gain;
● The offset voltage is 0.

Because AD8132 can be driven by a pure differential mode voltage, AD8132 can be widely used in various Single Input Circuits with differential mode output.
In the case of zero input in a single input circuit with a differential mode output, RG should be connected to the reference end (here the reference end is assumed to be local ). Figure 2 shows a basic circuit in which AD8132 uses two feedback circuits.

For feedback networks, the following feedback factors can be obtained:

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Among them, β1 refers to the feedback factor at the driving end, while β2 refers to the feedback factor at the voltage end. Each feedback factor can be 0 ~ Any value between 1. The general formula for circuit gain is:

G = 2 (1-β1)/(β1 + β2)

4.2 A/D Drive

The latest high-speed A/D converter is usually A single input, but also A differential mode input. Therefore, the drive of the/D converter should be able to convert A single input signal to A differential mode signal, and be able to translate the common mode output voltage level, and should be low-distortion and low-noise. For such A/D converter, AD8132 is the most suitable driver. Figure 3 shows the driving circuit of the/D converter AD9203 with 10-bit 40MSPS driven by AD813210.
In Figure 3, both AD8132 and AD9203 are driven by a 3 V single power supply. The AD8132 uses a 1VP-P signal as the input signal, and uses a voltage divider to adjust the VOCM voltage value to the voltage midpoint. Therefore, VOUT and dm also obtain the voltage midpoint value, which is within the common mode output range of AD9203. A single-stage differential filter is used between A/D and the drive to filter out part of the noise. Each A/D input end is composed of a dc signal of 0.5VP-P (range: 1.25V ~ 1.75 V) driver.

4.3 balanced cable driver

When driving a twisted pair cable, it is usually designed to transmit a differential mode signal. This is because the transmission line usually uses a balanced cable. Therefore, using a differential mode signal transmission can minimize the attenuation of the signal in the line.
The induction electric field is usually restricted in the area surrounded by two twisted cables, and generally no radiation occurs. The magnetic field caused by current in the cable may bring radiation, but the cable can often limit the electromagnetic radiation. Because the adjacent cable has the opposite magnetic polarity. If the two stranded cables are tight enough, these small magnetic loops will contain the vast majority of magnetic flux. Therefore, the effect of the far magnetic field can be ignored.
Any imbalance of the differential mode signal will act on the cable in the form of a common mode signal, which is equivalent to driving a single cable with a common mode signal. In this case, the cable is equivalent to an antenna. Therefore, in order to minimize the radiation, when the difference mode signal acts on the cable, the difference mode signal must be well matched. The common mode feedback loop of AD8132 helps to minimize the common mode voltage at the output end, so it can be used as a drive to balance the differential mode line. Figure 4 is a differential mode signal receiver using AD830 as the unit gain. It can be used to drive a cat5 cable of 10 meters in length.

4.4 sending balancer

As long as it is a wire, regardless of the length, it will degrade the information carried by it, especially the high-frequency signal. One compensation method is to use a balanced compensation circuit to stimulate high-frequency signals in the transmission circuit to weaken the signal attenuation at the ultimate receiver of the cable. At a higher frequency, decreasing the RG Impedance Value in the feedback network can increase the high-frequency gain of the circuit. Figure 5 shows the frequency pull-up circuit of a sending Line driver with a 2 gain. RG adopts 10pF capacitor shunt.

4.5 low-pass differential Filter

AD8132 can be applied to various types of Active Filter networks. They have A single input end and A differential mode output end, and provide the deprecated function when driving the differential A/D converter. Figure 6 shows a low-pass multi-feedback filter. Its Working part includes two-level filtering, and an additional filtering network at the output end. Its passband frequency is as high as 1 MHz.

4.6 High common mode output impedance amplifier

Change the line connection of VOCM to improve the common mode output impedance. In the circuit shown in Figure 7, VOCM is driven by a divider to adjust the common mode output voltage. At this time, the signal at the receiving end of a 10 m cat5 twisted cable is the midpoint of the voltage. All common-mode signals caused by noise will appear at the sender and must be absorbed at the sender. Therefore, the transmitter is required to have sufficient common-mode output range to completely absorb the common-mode signals produced in the cable. This is what people sometimes call the circuit "transformer effect ". However, the main difference between this circuit and the transmitter is that AD8132 also transmits DC signals, while the transmitter does not transmit DC signals.