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• TI Thinks Resolved

# TL026: TL026 ABOUT the AGC pin and the REF OUT pin issue

Part Number: TL026

Hi Team,

The customer is using TL026.  The VCC+ pin(6 pin) is 12V.  The VCC pin (3 pin) is GND.

Now the customer cannot provide the schematic for me. Once he provides the schematic for me, I will upload.

His design is like this:

(1) . A 6V bias voltage are input to both the IN+ pin and the IN- pin. And Another 200KHZ,30mVpp sine wave is input to the IN- pin at the same time.

(2). Then the customer tests the REF OUT pin voltage. It is about 7.31V. And this voltage remains the same.

(3). After the customer inputs the different voltage values to the AGC pin, then he tests the OUT- pin voltage. Please check the below informatio:

The AGC pin voltage value     The Vpeak voltage of the OUT- pin

0~7.26V                                             668mV

7.26V~7.428V                         FROM  668mV TO 38.8mV

7.452V~7.637V                     FROM 51mV TO 207mV

7.6387V~7.855V                FROM  207mV TO 68mV

>7.855V , the voltage waveform of the OUT- pin is messy.

Q1: When the VCC+ pin(6 pin) is 12V.  The VCC pin (3 pin) is GND, what is the voltage range for the REF OUT pin? Is 7.31V OK?

Q2: What is the voltage range for the AGC pin?

Q3: As the datasheet description, when the AGC pin voltage is less the REF OUT pin voltage 180mV, the gain is the max value.

When  the AGC pin voltage is higher than the REF OUT pin voltage 180mV, the gain is the min value. Is my understanding correct?

If my understanding is correct, for the customer's issues, why does the voltage of the OUT- pin increase first, then it reduce?

This is not consistent with the datasheet.

Q4: The customer would like to control AGC pin voltage to determine the gain. Is this idea correct?

Best Wishes,
Mickey Zhang
Asia Customer Support Center
Texas Instruments

• Mickey,
A1) 7.31 is close to low side. minimum is 7.3V. Typical is 7.4V (I just added 6V to what datasheet says).
A2) Normal AGC voltage is VREF +/-180mV; 7.13 to 7.49V per (dAVD Change in voltage amplification spec, typical 50dB)
A3) Your thinking is correct, I have never needed to verify figure 5 result before, but will do so this week.
A4) Yes.

What is the output load? How is it connected?

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

• In reply to Ron Michallick:

Mickey,

The AGC function works in a controlled manner when the output difference is used, [OUT+] - [OUT-].

This is gain vs. VAGC - VREF

Here is the gain using the output as single ended.

At low AGC voltage, OUT+ has twice the gain of VOUT-

The VOUT- gain is not monotonic , but VOUT+ is monotonic.

At high AGC voltage, both outputs are opposite phase and equal.

The OUT+ signal by itself may (or may not) work for your application.

For best performance both outputs should be used.

Regards,
Ronald Michallick
Linear Applications

TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).