Chopper Differential Difference Amplifier

There are a lot of researches related to instrumentation amplifier (IA), some had three-op-amp instrumentation amplifiers [23], the other had used current amplifiers [24], and in this design, the chopper differential difference amplifier (CHDDA) [18-19] is used to amplify biomedical signals.

As the biomedical signals are weak and easily affected by external noise, to obtain

17

accurate signals, the first stage is to use a CHDDA to amplify biomedical signals. In the past, the common approach is to use a three-op-amp IA. However, it not only requires larger area and consumes much more power, but also is prone to reduce common mode rejection ratio (CMRR) by fabrication mismatch. In other way, although the area of current feedback amplifier is not large, the power of it is a little too much in biomedical field. The most important reason is biomedical signals are longtime-observed weak signals, so chopper technology is used to amplify the biomedical signals with low power consumption.

There are some features of chopper such as low noise, low power, high tolerance…etc. But the most important feature is its’ low noise, the chopper circuit converts low frequency noise into high frequency one. Then the high frequency noise can be removed by a low pass filter (L.P.F.) and the biomedical signals can be converted back to their original frequency. Biomedical signals are only changed in form, from continuous-time signals to discrete-time signals, and the sample frequency is much higher than frequency of biomedical signals, so the biomedical signals are like almost the same as they were. Moreover, biomedical signals don’t change frequently, so the chopper circuit has no influence on real biomedical signals, the data that doctors need to make judgments.

The CMRR of general IAs is affected easily by the mismatch of resistors, capacitors and transistors. In addition, the influence at issue is serious. The differential difference amplifier (DDA) which is shown in Figure 2.5 can modify this defect, because CMRR is only related to the mismatch of the input ports. A DDA just requires a single active amplifier and two capacitors to set the gain, so the influence of the mismatch issue on the gain is little.

In this design, there are four inputs, Vnn, Vnp, Vpp and Vpn, using differential

18

difference pair technology. Vnn and Vnp are connected to input biomedical signals, Vpp is connected reference voltage and Vpn is connected to the feedback circuit.

Inputs are connected to chopper circuits, what they display is as the boxes in Figure 2.5, to convert low frequency noise to high frequency noise. The M1~M4 are input pairs and M5 and M6 supply current for the circuit. Then M1~M4 convert signals from voltage type to current type, M7 and M8 convert signals back to voltage type and convey them to M9~M14 to be amplified. Finally, the boxes are the chopper circuits as shown in Figure 2.6. It is necessary to provide two non-overlapping clocks for chopper circuits, the clocks will be discussed in section 2.8. The feature of chopper circuit is to cover a normal signal to a high frequency and discontinue signal.

In the biomedical domain, physiological signals show no large diversification, so discontinue signals may not affect final result. And the most important is another feature, converting a normal signal to a high frequency signal. This way, low frequency noise such offset noise can be converted to high frequency noise and removed by a low pass filter which will be discussed in section 2.5.

Figure 2.5 The circuit of chopper differential difference amplifier

19

Figure 2.6 The circuit of chopper technology

Figure 2.7(a) The feedback architecture of CHDDA (b) Using Capacitor feedback to replace the resistor feedback

Biomedical signals are weak signals, so it is necessary to amplify them until reaching an appropriate level. For signal processing in the back-end, the gain of CHDDA is 40dB and the circuit is shown in Figure 2.7 (a). In Figure 2.7 (a), the gain is decided by the resistors, R1 and R2, and Equation 2.1 shows the gain. The characteristic of this design is system on chip, so all components are on a chip include resistors. Just like the above mentioned, it is necessary to control the power consumption for long time observation. The current must be low enough to control the power consumption. This way, the resistors may become too large, making it difficult to implement on the chip. To solve the problem, the design replaces resistors by capacitors as shown in Figure 2.7 (b). The Equation 2.2 can be used to determine the

20

gain. Finally, the sizes of all transistors are listed in Table 2.1.

2.1 2.2 Table 2.1 The sizes of transistors

Devices W/L (um)

The chopper differential difference amplifier (CHDDA) which is DDA using the chopper technology also has some features such as high tolerance to mismatch of input ports, high CMRR, and low input offset noise. And the CHDDA uses only one active amplifier, so the area is smaller than a 3OPIA which uses three active amplifiers.

When applying it in the biomedical field, it is usually set on patients’ bodies. It is not only inconvenient but also uncomfortable for patients. As for the portable AFE IC, it is necessary to think about the convenience of patients. The movement of patients may lead to a large DC offset so that the AFE circuits may enter the saturation region.

In order to solve this problem, the design makes an impendent reference [25]

voltage for CHDDA. This impendent reference voltage is composed of capacitors as show in Figure 2.8. First, capacitors are used at the input of the CHDDA to avoid any DC offset and then giving inputs some transistors and another reference voltage. This way, it creates a new independent DC offset to input by diode-connected MOS which

21

controls the level of DC offset to keep the circuit work normally.

Figure 2.8 The control circuit of impendent reference voltage

The modified CHDDA with an independent and stable DC offset circuit is shown in Figure 2.9. The transistors M1~M4 provide new impendent reference voltage to CHDDA and capacitors C1 and C2 remove the DC offset voltage. In another way, the transistors M5 and M6 provide the path of current, capacitors C3 and C4 determine

Figure 2.9 The circuit of chopper technology Table 2.2 The sizes of devices

Devices Value Devices W/L (um)

C1~C2 5 p M1~M4 1/1

C3 0.1p M5~M6 1/1

C4 10p

22

The simulation results are shown below in Figure 2.10, Figure 2.11, Figure 2.12, Figure 2.13, and Figure 2.14. The yellow line represents the input signal, black line represents the TT process; red line represents the FF process; blue line represents the SS process; green line represents the SF process; purple line represents the FS process.

Figure 2.10 Frequency response (a) Pre-Layout-simulation (b) Post-layout-simulation

Figure 2.11 Common-mode rejection ratio (CMRR) (a) Pre-Layout-simulation (b) Post-layout-simulation

23

Figure 2.12 Input common-mode range (ICMR) (a) Pre-Layout-simulation (b) Post-layout-simulation

Figure 2.13 Slew rate (SR) (a) Pre-Layout-simulation (b) Post-layout-simulation

Figure 2.14 Input noise (a) Pre-Layout-simulation (b) Post-layout-simulation

24

Figure 2.15 Function simulation of CHDDA (a) Pre-Layout-simulation (b) Post-layout-simulation:

Figure 2.16 Function simulation of modified CHDDA

Figure 2.17 Function simulation of modified CHDDA

25

In Figure 2.15(a), the blue line is input signal and the pink is output signal, and the green line is input and blue line is output in the Figure 2.15 (b). The Figure 2.16 show the independent reference voltage simulation, the blue line is input signal and the red line is output. In Figure 2.16, the output will follow the DC level independent reference voltage and amplify the input normally as show in Figure 2.17. In this way, the design will transform the signal from low frequency to high frequency without distortion.

The parameter simulations of DDA are shown in Table 2.3, Table 2.4, Table 2.5, Table 2.6, Table 2.7, and Table 2.8.

Table 2.3 0℃ Pre-Layout-Simulation

3.7778E+05 4.8505E+05 7.8542E+05 8.6699E+06 4.1905E+05

ICMR（V） 0.591~1.02 0.593~1.02 0.58~1.03 0.502~1.1 0.635~0.968

CMRR（dB） 75.7 69.75 76.4 76.36 59.6

26

Table 2.4 25℃Pre-Layout-Simulation

Table 2.5 80℃ Pre-Layout-Simulation noise (nV)

Output range 0.0117~1.09 0.016~1.09 0.009~1.08 0.038~1.15 0.0043~1.06

TT FF SS SF FS

3.5643E+05 5.8187E+05 5.7070E+05 9.4152E+06 4.5284E+05

ICMR（V） 0.602~1.01 0.621~0.983 0.613~0.99 0.112~1.06 0.674~0.944

CMRR（dB） 75.88 70.37 76.71 76.27 61.08

Output range 0.0117~1.09 0.016~1.09 0.009~1.08 0.038~1.15 0.0043~1.06

TT FF SS SF FS

Gain（dB） 72.8 65.7 74.0 70.5 68.3

PM（°） 77.9 69.7 100.4 51.1 77.5

Unity-gain 5.2323E+05 7.7161E+05 3.5755E+05 9.6572E+06 3.7866E+05

27 ICMR（V） 0.7~1.11 0.73~1.08 0.717~1.0

9

ICMR（V） 0.587~1.03 0.606~0.992 0.602~1 0.22~1.1 0.641~0.956

CMRR（dB） 76.17 71.83 77.07 75.89 64.04

Output range 0.0144~1.11 0.019~1.11 0.018~1.10 0.040~1.16 0.0058~1.07

28

29

In this design, it is necessary to design an operation amplifier for low pass filter and adjustable gain amplifier. Because biomedical signals are weak signals, the operation amplifier must be of low noise. In another way, the physiological signals have to be observed for a long time to ensure the diagnoses of diseases are correct.

For long time observation, the power consumption must be low enough to maintain system’s normal operation. The operation amplifier of is low noise and low power in this design as shown in Figure 2.18, Figure 2.19, Figure 2.20, Figure 2.21, Figure 2.22, and Figure 2.23.