Agc/Error Amplifier Circuit - Integra Cusa Excel Service Manual

Ultrasonics Board–Ultrasonic Generator
7-10
handpiece is connected, compensates for some of this loss for the 35.75 kHz
circuit. The -3 dB point of the RF filter is at about 100 kHz to maximize RF losses
at 500 kHz and above, and to minimize passband phase.
The active filter is an analog state-variable structure "universal active filter" IC.
R95 and R96 determine the bandpass filter gain. R63, R75 and RV6 determine the
center frequency for the 23 kHz circuit (or R62, R65, and RV7 for the 35.75 kHz
circuit) as selected by the analog switches U10A, B, D, and U4C.
The BPF center frequency ensures that midband phase and gain are as flat as
possible, but losses at the undesired longitudinal modes are maximized. For the
23 kHz circuit, the first undesired mode occurs at about 26 kHz; for the 35.75 kHz
circuit, it occurs at approximately 43.8 kHz. Handpiece/tip length primarily
determines this mode, although handpiece/tip diameter also affects it. Loss at
these frequencies must come from two sources:
(Most obvious) BPF loss at these frequencies. However, the AGC loop gain
1.
may, in most instances, be sufficient to negate this loss.
Phase. If the phase is sufficient that the only operating point possible at this
2.
mode requires a very high gain, then this will not be a preferred mode.

AGC/Error Amplifier Circuit

The AGC function consists of the following components:
• A root mean square (RMS) to DC circuit centered around U23
• A handpiece (HP) amplitude input buffer centered around U22A
• An amplitude error amplifier centered around U22B
• A multiplier centered around U15.
C106 programs the RMS-to-DC conversion accuracy and low frequency pole.
R146, R147, C102, and C103, together with the internal buffer amplifier of the
IC, form a two-pole Sallen and Key low pass (LP) filter to remove high-frequency
noise. A time constant (the product of C106 times the internal resistor in the
RMS-to-DC converter) determines how fast the generator reacts and recovers
from load and operating point condition changes.
The amplitude input buffer, U22A, filters (R100 and C73) and inverts the
handpiece amplitude signal. The active rectifier circuit (CR16 and CR17)
prevents positive valued amplitudes that could cause loop instability at the error
amplifier.
The error amplifier, U22B, subtracts the RMS-to-DC signal from the handpiece
amplitude signal. The residual signal is the loop gain value sent to the multiplier,
U15. Two factors determine the steepness of the gain of this error amplifier:
• The input signal levels
• The minimum dynamic range required for the handpiece stroke levels under
load and start-up conditions.
RV8 sets the gain of the amplifier, while RV9 sets the gain for the feedback signal
levels.
The control function has an offset and a gain which limit the operating range
capabilities (start-up and load) of the handpiece. At minimum level settings the
control function must have a start-up gain of at least two times or greater the
CUSA EXcel Ultrasonic Surgical Aspiration System Service Manual