Imagine trying to measure a current so tiny that it’s almost invisible. How is that even possible? How do we even convert that current signal into something that is useful? With Transimpedance Amplifiers (TIAs), measuring and converting these tiny current signals is a piece of cake!
TIAs convert the current output of sensors, such as photodiodes, into a voltage signal compatible with most electronic circuits and instruments. They are commonly used in optical communication systems, imaging systems, medical devices, and many other applications.
For these applications, the TIA output voltage must be accurate, undistorted, and stable (i.e., non-oscillating) to preserve the integrity of the original signal. Additionally, ensuring stability prevents potential damage to the instrument or system, particularly in the subsequent stages following the TIA.
As discussed in Part 1 of the Amplifier Stability Essentials blog series, the EVAL-KW4503Z utilizes one channel of the ADA4510-2 for the TIA Stability KWIK demo. It allows the user to learn how to stabilize an op amp configured as a TIA by selecting from three compensation options via a jumper-configurable feedback network on J1.
This blog explores the key considerations in designing the TIA circuit on the EVAL-KW4503Z, along with basic compensation techniques to ensure its stability.
Figure 1. TIA Stability KWIK Demo
Note: The ADALM2000 Scopy Configuration file for this demo can be found on Stability KWIK Lecture + Lab (Wiki)
Amplifier Selection
It’s important to choose an op amp that has the following characteristics when designing a TIA circuit:
- Low Input Bias Current – to minimize added current on the original input current signal and reduce current-to-voltage conversion errors.
- Low Input Offset Voltage – to minimize output voltage error.
- Low Op Amp Input Capacitance (Common Mode and Differential) – to reduce the overall or total input capacitance.
- Adequate Gain-Bandwidth Product – determines how fast the op amp can respond to input signals. For high-speed photodiode signals, the higher the GBP, the better.
- High Open-Loop Gain – for lower distortion and better linearity.
For EVAL-KW4503Z, one channel of ADA4510-2 was used and configured as a TIA. ADA4510-2 was selected because it possesses all the key characteristics discussed earlier.
Transimpedance Gain
The transimpedance gain of an op amp configured as a TIA can be computed as the ratio of the output voltage to the input current coming from the photodiode.
RF = VOUT / IIN
On EVAL-KW4503Z, a 10kΩ feedback resistor (RF) was used to amplify approximately 45µA of signal current from the photodiode.
Total Input Capacitance
To estimate the approximate total input capacitance, the following formula was used:
CIN = CPHOTO + CCM + CDIFF + COTHERS
Where:
- CIN = Total Input Capacitance
- CPHOTO = Photodiode Capacitance
- CCM = Op Amp Common-Mode Input Capacitance
- CDIFF = Op Amp Differential Input Capacitance
- COTHERS = Capacitance caused by PCB parasitic, RF’s self-capacitance, etc.
For the TIA circuit of EVAL-KW4503Z, the following values were used:
- CPHOTO = 70pF (VBPW34S – Vishay Semiconductors)
- CCM = 2pF (ADA4510-2)
- CDIFF = 20pF (ADA4510-2)
- COTHERS = 1.5pF (PCB parasitic and RF’s self-capacitance)
- CIN = 93.5pF
This total input capacitance will introduce a zero on the 1/β curve, which can be computed by using the formula:
Fz = 1 / (2π*RF*CIN) = 1 / (2π*10kΩ*93.5pF)
Fz ≈ 170kHz
Figure 2. Location of Fz on the 1/β curve (Ideal Closed-Loop Gain)
If not properly compensated, it can make the TIA circuit unstable. As shown in the simulation below, the point of intersection of the Aol and 1/β curve yields a 40dB/dec rate of closure. To stabilize the circuit, we need a rate of closure equal to 20dB/dec.
Figure 3. Simulating TIA Stability through LTSpice – Marginally Stable
Note: On the simulations, CCM and CDIFF have been removed from CIN because they are already included on the ADA4510 SPICE model. CPCB = COTHERS
Feedback Capacitance
To remove the unwanted oscillations in the output signal and stabilize the TIA circuit, an adequate value of feedback capacitance in parallel with RF is required. By using the formulas below, a 20db/dec rate of closure and 45° Phase Margin can be obtained.
FU = Op Amp’s Unity Gain Frequency (ADA4510) = 10MHz
FCOMP = √(FZ*FU) = √(170kHz*10MHz) = 1.3MHz
CF = 1 / (2π*FCOMP*RF) = 1 / (2π*1.3MHz*10kΩ ) = 12.24pF
Figure 4. Simulating TIA Stability through LTSpice – 45° Phase Margin
If we try to overcompensate the circuit and increase the feedback capacitance up to 47pF (CF2 on EVAL-KW4503Z), the Phase Margin will increase to >70°, as shown below. Notice also that the rate of closure of Figure 6 remains at 20db/dec.
Figure 5. Simulating TIA Stability through LTSpice – >70° Phase Margin
Figure 6. Simulating TIA Stability through LTSpice – >70° Phase Margin (Open-Loop Gain and new 1/β curve)
What’s Next
This blog explored the key considerations in designing an effective TIA circuit, highlighting essential compensation techniques to ensure stability. It enables us to accurately convert current into voltage while ensuring the protection of electronic equipment or instruments from potential damage.
Part 3 of the series will discuss the Capacitive Load Stability KWIK Demo. This will enable readers to learn how to stabilize an op amp driving a capacitive load, which could help protect the op amp from accidental damage. Additionally, it can protect the instrument or system where the op amp is used.
Read all the blogs in the Amplifier Stability Essentials series.