How to Use LMP7721 3-Femtoampere Input Bias Current Precision Amplifier: Examples, Pinouts, and Specs

The LMP7721 is a high-precision operational amplifier manufactured by Texas Instruments. It is specifically designed for applications requiring extremely low input bias current, high input impedance, and low offset voltage. With an input bias current as low as 3 femtoamperes (fA), the LMP7721 is ideal for interfacing with high-impedance sensors and performing precision signal conditioning.

• High-impedance sensor interfacing (e.g., photodiodes, pH sensors, and ion-sensitive electrodes)
• Precision instrumentation and measurement systems
• Medical instrumentation
• Charge amplifiers
• Low-leakage integrators
• High-accuracy current-to-voltage converters

The LMP7721 is designed to deliver exceptional performance in precision applications. Below are its key technical specifications:

The LMP7721 is available in an 8-pin SOIC package and a 5-pin SOT-23 package. Below is the pinout and description for the SOIC-8 package:

For the SOT-23-5 package, the pinout is as follows:

1. Power Supply: Connect the positive supply voltage (V+) and negative supply voltage (V-) to the appropriate pins. For single-supply operation, connect V- to ground.
2. Input Connections: Connect the signal source to the non-inverting (IN+) or inverting (IN-) input, depending on the desired configuration (e.g., non-inverting or inverting amplifier).
3. Output Load: Ensure the output is connected to a load within the amplifier's drive capability. The LMP7721 supports rail-to-rail output swing.
4. Bypass Capacitors: Place decoupling capacitors (e.g., 0.1 µF ceramic) close to the power supply pins to minimize noise and ensure stable operation.
5. High-Impedance Applications: To maintain the ultra-low input bias current, avoid contamination or leakage paths on the PCB. Use guard rings around high-impedance nodes and clean the PCB thoroughly.

The LMP7721 is well-suited for amplifying the small currents generated by photodiodes. Below is an example circuit for a transimpedance amplifier:

   Photodiode
      |
      |---- IN- (Pin 2)
      |     |
      |     Rf (Feedback Resistor)
      |     |
      |---- OUT (Pin 6) ----> Amplified Signal
      |
     IN+ (Pin 3) ----> Ground

The LMP7721 can be used with an Arduino UNO to measure small currents or voltages. Below is an example code snippet for reading the output of the LMP7721:

// Example: Reading LMP7721 output with Arduino UNO
// Connect LMP7721 output to Arduino analog pin A0

const int analogPin = A0; // Analog pin connected to LMP7721 output
float voltage = 0.0;      // Variable to store the measured voltage
float vRef = 5.0;         // Reference voltage of Arduino (5V for UNO)

void setup() {
  Serial.begin(9600); // Initialize serial communication
}

void loop() {
  int adcValue = analogRead(analogPin); // Read ADC value (0-1023)
  voltage = (adcValue / 1023.0) * vRef; // Convert ADC value to voltage
  Serial.print("Voltage: ");
  Serial.print(voltage, 3); // Print voltage with 3 decimal places
  Serial.println(" V");
  delay(500); // Wait for 500 ms before next reading
}

• PCB Design: Use a clean PCB layout with guard rings around high-impedance nodes to prevent leakage currents.
• Feedback Resistor: Choose a high-value feedback resistor (e.g., 1 MΩ to 10 GΩ) for transimpedance applications.
• Input Protection: Avoid exposing the inputs to voltages beyond the supply rails to prevent damage.
• Temperature Effects: Consider the operating temperature range and ensure the circuit components are rated accordingly.

1. High Noise in Output Signal

   • Cause: Insufficient power supply decoupling or noisy environment.
   • Solution: Add decoupling capacitors (e.g., 0.1 µF ceramic) close to the power supply pins and shield the circuit from external noise.

2. Unexpected Offset Voltage

   • Cause: PCB contamination or leakage currents.
   • Solution: Clean the PCB thoroughly and use guard rings around high-impedance nodes.

3. Output Saturation

   • Cause: Input signal exceeds the amplifier's input range or feedback resistor is too high.
   • Solution: Ensure the input signal is within the specified range and adjust the feedback resistor value.

4. Low Gain or Incorrect Output

   • Cause: Incorrect feedback resistor or wiring.
   • Solution: Verify the feedback resistor value and check the circuit connections.

Q: Can the LMP7721 operate with a single power supply?
A: Yes, the LMP7721 can operate with a single supply voltage as low as 1.8 V. Connect V- to ground for single-supply operation.

Q: What is the maximum feedback resistor value I can use?
A: The LMP7721 supports feedback resistors up to 10 GΩ, making it suitable for high-impedance applications.

Q: How do I minimize leakage currents in my circuit?
A: Use guard rings around high-impedance nodes, clean the PCB thoroughly, and avoid using materials with high leakage properties.

Q: Is the LMP7721 suitable for battery-powered applications?
A: Yes, with its low supply current and wide supply voltage range, the LMP7721 is ideal for battery-powered systems.