TL852

The TL852 is an economical sonar ranging receiver integrated circuit for use with the TL851 control integrated circuit. A minimum of external components is required for operation, and this amplifier easily interfaces to Polaroid's 50-kHz electrostatic transducer. An external 68-kΩ ±5% resistor from BIAS to GND provides the internal biasing reference. Amplifier gain can be set with a resistor from G1IN to GADJ. Required amplifier gain will vary for different applications. Using the detect-level measurement circuit of Figure 1, a nominal peak-to-peak value of 230 mV input during gain step 2 is recommended for most applications. For reliable operation, a level no lower than 50 mV should be used. The recommended detect level of 230 mV can be obtained for most amplifiers with an R1 value between 5 kΩ and 20 kΩ

Digital control of amplifier gain is provided with gain control inputs GCA, GCB, GCC, and GCD. These inputs must be driven synchronously (all inputs stable within 0.1 µs) to avoid false receive output signals due to invalid logic counts. This can be done easily with the TL851 control integrated circuit. A plot showing relative gain for the various gain steps versus time can be seen in Figure 2. To dampen ringing of the 50-kHz electrostatic transducer, a 5-kΩ resistor from G1IN to XIN is recommended.

An external parallel combination of inductance and capacitance between LC and  V_CC provides an amplifier with an externally controlled gain and Q. This not only allows control of gain to compensate for attenuation of signal with distance, but also maximizes noise and sidelobe rejection. Care must be taken to accurately tune the L-C combination at operating frequency or gain and Q will be greatly reduced at higher gain steps.

AC coupling between stages of the amplifier is accomplished with a 0.01-mF capacitor for proper biasing.

The receive output is normally held at a low level by an internal 1-µA current source. When an input of sufficient amplitude is received, the output is driven alternately by the 1-µA discharge current and a 50-µA charging current. A 1000-pF capacitor is required from REC to GND to integrate the received signal so that one or two noise pulses will not be recognized.

XIN provides clamping for the transformer secondary when used for transducer transmit drive as shown in Figure 4 of the SN28827 data sheet.

The TL852 is characterized for operation from 0°C to 40°C.