Amplifier AD604 and its medical application of ultrasound system

AD604 is Analog Devices (AD company) product.

And similar products, compared with the ultra low noise AD604, high precision, adjustable gain, and the gain in DB (dB) and gain control voltage proportional to the characteristic. And medical ultrasound instrument time gain control (TGC) circuit requires their gain and control voltage exponential relationships, gain of decibels (dB) and control voltage into a linear relationship. Therefore, in this regard, the AD604 is an ideal ultrasound TGC amplifier, it can effectively reduce the fed A/D converter dynamic range of the signal. AD604 's main characteristics are as follows:

· Low input noise, the maximum gain, the voltage and current noise respectively · bandwidth (-3dB) 40MHz; · has two independent gain channel and gain of decibels (dB) and gain control of the proportional voltage; · the gain for each channel can be programmed.

The preamplifier 14 dB gain available when 0 ~ + 48dB between adjustments; in pre-amplifier 20dB, gain in + 6 ~ + 54dB adjustable between; · has 300 k Ω input impedance; · variable gain range is 20 ~ 40dB/V; · on temperature and voltage changes, its gain very stable; · to gain control of single you polarity; · when gain exceeds the maximum limit, the low power will automatically power off; · to direct drive A/D converter. 1 pin function

AD604 adopted 24 foot packaging and DIP, SOIC SSOP and three package, the pins are arranged as shown in Figure 1.

The PIN function as follows:

PAI1/PAI2: preamplifier is input; PAO1/PAO2: preamplifier output; FBK1/FBK2: preamplifier feedback-end; COM1/COM2: signal ground; when it received positive power preamplifier channels are closed;-DSX1/-DSX2: differential attenuator signal input negative side; + DSX1/+ DSX2: differential attenuator signal input is end; VGN1/VGN2: gain control of inputs and power off-side.

Grounding, the attenuation of the channel is closed; otherwise, as the positive voltage increases, gain will gradually increase; VREF: two-channel gain control. When the voltage of + 2.5V, gain 20dB/V, and when the voltage of + 1.67V, gain 30dB/V; VOCM: output signals of the common mode signal control client. To determine which part of the circuit DC signal voltage; values in OUT1 OUT2 signal output/:; VPOS/end: received positive VNEG/negative power supply; GND2 ground GND1/:.

2 internal structure and working principle

AD604 is a dual-channel variable gain amplifier.

It's every channel is represented by a low-noise preamplifier and a variable gain amplifier (XAMP). At the same time a XAMP consists of high-precision controlled differential attenuator, a gain control unit, a fixed-gain amplifier and a discrete components R3, R4, composed of VOCM common-mode voltage control units. Its principle is shown in Figure 2. AD604 of each channel provides a range of variable gain 48dB.

2.1 preamplifier

AD604 every channel has a high-performance preamplifier, through feedback loops on an external resistance to magnification control + 14dB ~ + 20dB.

Preamplifier internal circuit shown in Figure 3. Which R5, R6, R7 is the preamplifier gain control resistance. The size of specific gain by FBK1 and resistance between PAO1.

2.2 differential ladder network (attenuator)

Preamplifier output can be used as indices of differential input amplifier for DB linear gain.

The Attenuator is a 7 bands R-1.5R ladder network. Each order for attenuation, the final total 6.908dB attenuation as 48.356dB.

2.3 gain control

AD604 linear gain control is achieved through the VGN-end.

Users can add their own needs change voltage input to VGN for a change of gain. VGN-end of input resistance is 2 M Ω.

In order to adapt to the different needs of the user to gain the AD604 also provides a gain control.

By adjusting the VREF input voltage can adjust the gain of grades: VREF voltage from 2.5 ~ 1.20 ~ 25V correspond 40dB/V's gain. For 20dB/V gain adjustment, VGN 0.2 to 1.2V; for 40dB/V gain adjustment, VGN 0.4 to 2.4V. Current Reset zoom to 14dB, you can press the following formula to calculate the gain:

2.4 fixed-gain amplifier

This level of magnification is composed by an operational amplifier feedback circuit.

Feedback amplifier output in all the way to enter as a feedback; the other way is as a differentiator for input (see Figure 2).

This part of the overall gain amplifier: G total = VOUT/VATTEN = [(R1 + R2)/R2] gm1/gm2.

Of these, VOUT is output voltage, VATTEN is attenuator readout, (R1 + R2),/R2 = 42 = 1.25 gm1/gm2. Therefore, the totalGain 52.5 (i.e. 34.4dB).

3 AD604 in medical application of ultrasonic equipment

When medical ultrasound instruments issued by ultrasonic wave propagation in the human body, the energy will be absorbed by the human organism.

With the detection depth increases, the ultrasonic energy will gradually fade. In order to make different depth organization interface back to the same amplitude, the different depth of echo signal attenuation zoom levels to achieve sound path compensation, that is, the need to gain receiver with the scanning time increases because the sound from the darker parts of the interface reflection of echo signal amplification is large, but away from the transducer near reflected signals in time on an earlier arrival of echo the magnification is small. In ultrasonic diagnostic instrument, usually using TGC (Time Gain Compensation) depth time gain compensation circuit, i.e., a certain voltage curves to control amplifier gain, so that different depth of ultrasonic echo to get different magnification, which played the role of compensation.

Shown in Figure 4 is a AD604 driven AD9050 (10-bit, 40MSPS ADC) ultrasonic gain compensation circuit.

When AD7226 d/a converter and other microprocessor interface, should read magnification digital conversion to analog, and then put this analog as AD604 gain control signal input (connected to its VGN-end), thereby achieving gain control. After the signal attenuation compensation AD604, then through the filter and AD9631 (low distortion, low noise, high speed operational amplifier) to become an effective importation of ADC. Operational amplifier output and input from the partial ADC in AC-coupled, by AD9050 a/d converter with sample rate conversion for 40MSPS modulus.

The programme addresses the medical ultrasound measurement of soft tissue by acoustic path leading to the echo signal linear compensation issues.

Unlike traditional discrete components compared to the circuit, the programme has the circuit is simple, stable and reliable TGC control signals as well as the adjustment flexibility characteristics, accurately compensate for ultrasonic attenuation in the human body, and thus to control system for high-speed digital offers a new and reliable method.

Reference documents

1．Analog Devices．AD604 Data Sheet

2. wanming acquisition, bian Cheng Cheng jingzhi. medical ultrasonics. Xi'an Jiaotong University Press, 1992.10

3. Guan Li Ji Seung-Hoon, Ray. ultrasonic equipment and maintenance.

People's medical publishing house, 1983.12

4．Chebli，R．；Kassem，A．；Sawan，M．＇Integrated front－end preamplifier dedicated to ultrasonic receivers＇Electronics，Circuits and Systems，2001． ICECS2001．The 8th IEEEInternational Conference on，2001，Page（s）：1103－1106 vol．3