Valleylab CUSA EXcel-8 - Agc;Error Amplifier Circuit

Ultrasonics Board—Ultrasonic Generator
and RF filter consisting of R48, R49, R67, R68, C43, C46, and C53. The filter
and pad pass the differential mode ultrasonic frequencies at about 16 dB
of midband loss due to the large feedback voltage swings of up to 90 Vp-p
of the 23 kHz handpiece/tip. The gain set resistor R66, selected by the
analog switch U10C when the 35.75 kHz 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:
1. (Most obvious) BPF loss at these frequencies. However, the AGC loop
gain may, in most instances, be sufficient to negate this loss.
2. Phase. If the phase is sufficient that the only operating point possible at
this 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:
* Aroot mean square (RMS) to DC circuit centered around U23
i
* A handpiece (HP) amplitude input buffer centered around U22A
e Anamplitude error amplifier centered around U22B
e 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 4
of this error amplifier: |
7-10 CUSA EXcel-8 System Service Manu