Application Note for High speed op amp--matching of high-speed op amp

In the field of medical health, the application of bio-impedance measurement technology has a wide prospect. According to human body or biological tissue impedance change, to judge the physical state of human body, cell electrical characteristics of small changes, typical operation for human sleep respiration monitoring, biological tissue early detection of cancer and other fields. As the shape and structure of the organism to be measured, it is necessary to measure the analog front end with sufficient accuracy, large dynamic, and high speed (generally considered as high speed above 10Msps). High-speed OP amps are widely used in differential amplification, buffering, line-driven, and drive ADCs. Precise design helps improve system linearity, resolution, and product reliability.

Analysis system performance is divided into three parts: analog front-end measurement, equivalent to the oscilloscope differential probe, may have up to several meters of line transmission, digital partial op amp buffer amplification, ADC sampling. The cost of the ADC in the system is the highest, the op amp design should not be the system's short board, can be from the equivalent bandwidth, equivalent effective bit width, harmonic distortion, noise and stability of the targeted analysis. The impact of differential op-amp output imbalance on system performance. When we choose Precision op amp, we always hope that the common mode rejection ratio is large enough to eliminate the power frequency interference, the common mode noise of the supply coupling, and the coupling noise on the input line. If the differential op-amp of the FIG1 is not well matched, the CMRR will be significantly reduced, and the imbalance will cause the odd harmonics to increase, if the reference signal is a mono sine signal, coherent demodulation, the harmonics directly reduce the system's signal-to-noise ratio.

The FIG1 differential amplifier's mismatch model feedback factor can be expressed as

Can get an expression of the output differential signal

If the resistor is not balanced, i.e. β1≠β2, the output differential mode voltage depends on the VOCM, which will generate offset and excessive noise in the differential output. In the actual situation, there are different line impedance, different resistance accuracy, different amplifier cascade and other scenes, the task of resistance matching is to precisely ensure the β1 =β2, reduce the offset effect.

The first step is the input impedance calculation of the differential op amp. The signal coming from the general line is a single-ended signal, so assume that the system

A single-ended input differential output architecture. Adopt a formula.

As shown in the FIG2 model, the calculated output G into the impedance of 267ohm.

Fig2 input impedance calculation for single-ended inputs

Considering that the line cable is generally a single-ended 50ohm, in parallel to the op-amp input port to ground resistance RT, the input resistance is reduced to about 50ohm, to get
。 If our source is a 2Vpp voltage signal, and the 50ohm characteristic impedance of the cable drive, op amp input impedance also matched to 50ohm, then this circuit is a good match, such as FIG3.

Fig3 50ohm system source and load matching

The matching elements of the fig3 are complete, but due to the source impedance, the source impedance needs to be iterated. So the second step is to calculate the equivalent thevenin parameters of RT and source impedance, which can simplify the calculation. Thevenin equivalent open circuit voltage Vth is 1.1Vpp, the equivalent resistor rth 50ohm and 61.5ohm shunt resistance 27.6ohm, such as FIG4.

FIG4 equivalent Thevenin circuit

We want the 1VPP output voltage, but the 1.1V thevenin voltage at the output voltage produces a voltage of VOUT,DM = 1.1vpp∗ (200/227.6) = 0.97VPP. With the ideal 1Vpp voltage difference, you can adjust the feedback resistor to meet the requirements.

such as Fig5.

FIG5 Final 1VPP output

It is worth noting that the input resistance of the entire op amp has changed since the RF changes. Fortunately, when the closed-loop gain is 1 or 2 o'clock, this change is very small and the iteration can meet the matching requirements.

The effect of measured line impedance on differential op amp (test measurement).

Signal Integrity design requirements for high-speed digital-to-analog signals. In the schematic design phase should consider the power of the broadband filter, the impedance of the single capacitor will change with the increase in frequency, to the resonant frequency will appear perceptual, as shown in Fig6. Therefore, it is necessary to adopt a plurality of different capacitance bypass filters, the general low capacitance is close to the device pin.

fig6 Capacitor Impedance VS. Frequency

The parasitic capacitance of the OP amp input can cause potential instability and loop oscillation. In the case of Fig7, the capacitance at the op-amp input 1pF leads to a noticeable spike in the gain curve.

Fig7 parasitic capacitance caused by spikes

If there is no reference to the route, there will be a noticeable inductance effect. 25.4mm without reference to the line, can reach the parasitic inductance of 29nH, which is very considerable. Fig8 indicates that the parasitic inductance is present when the impact response is compared.

FIG8 shock response compared with parasitic inductance

The layout of high-speed op-amp should reduce the parasitic inductance and capacitance of the two efforts, such as the line has a good reference, in the vicinity of the pin to knock out the formation, can be guaranteed to reduce the plane parasitic input capacitance, feedback loop line as short as possible. These guidelines are not only high-speed op amp, but also suitable for other high-speed layout, but the specific problem has a specific analysis method.

Application Note for High speed op amp--matching of high-speed op amp