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Download May 2001 1MW Transimpedance Amplifier Achieves Near-Theoretical Noise Performance, 2.4GHz Gain Bandwidth, with Large-Area Photodiodes
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DESIGN IDEAS 1MΩ Transimpedance Amplifier Achieves Near-Theoretical Noise Performance, 2.4GHz Gain Bandwidth, with Large-Area Photodiodes by Glen Brisebois transimpedance circuit’s noise gain, which applies to voltage noise but not to current noise or resistor noise, rises drastically with frequency (noise gain = 1+ ZF/XC). As a sample calculation, a 500pF photodiode has an impedance of 3.2k at 100kHz, giving a 1MΩ transimpedance circuit a noise gain of 314 at that frequency. The theoretical noise floor of the 1M resistor is 130nV/√Hz (at room temperature), so any input voltage noise above 0.41nV/√Hz (130nV/√Hz/314) will overtake the resistor noise at 100kHz. Discrete JFETs are available with ultralow voltage noise but they have high input capacitance (75pF max for the IFN147). Serendipitously, the input capacitance of the JFET is relatively insignificant compared to the 500pF of the example large-area photodiode. Although the capacitance of the JFET increases the overall noise gain slightly, its much lower input voltage noise is well worth the slight increase in total input capacitance and noise gain. Table 1 and Figure 3 ZF R4 1M C4 1pF 5V C5 1pF R5* 210Ω + INTERFET IFN 147 (972 487-1287) XC PHOTO* DIODE LT1806 – R6 4.7k R1 10M OUT C3 0.1µF 5V R3 10k + C1 100pF LT1793 – C2 0.1µF *SEE TEXT VS = ±5V R2 10M Figure 1. Ultralow noise, 2.4GHz gain bandwidth, large-area photodiode amplifier 2500 OUTPUT NOISE DENSITY (nV/√Hz) The circuit of Figure 1 shows an ultralow noise transimpedance amplifier connected to a large-area, high capacitance photodiode. The LT1806 is used for its high gain bandwidth and low noise. The IFN1471 ultralow noise JFET operates at its IDSS (VGS = 0V) with a typical transconductance of 30mS. With its source grounded, the JFET and its drain resistor, R5,2 set up a voltage gain of about six. The combination of the ultralow noise JFET gain stage and the LT1806 low noise amplifier achieves ultralow input noise performance. The circuit’s input voltage noise was measured at 0.95nV/√Hz. Figure 2 compares the noise performance of the circuit with that of a competitor’s monolithic 6nV/√Hz JFET op amp, both with a 680pF capacitive source. Why is it necessary to have both low voltage noise and low current noise to achieve low total noise in a large-area photodiode transimpedance amplifier? Because the 2000 xxx655 1500 COMPOSITE FIGURE 1 1000 500 0 0 20k 60k 40k FREQUENCY (Hz) 80k 100k Figure 2. Composite amplifier vs competing xxx655 op amp show the bandwidth and noise performance achieved with several large-area photodiodes (and include a small-area SFH213 for comparison). Note that large area detectors also place extra demands on the gain bandwidth of an amplifier. The final case in Table 1 shows a 1MΩ transimpedance amplifier with 650kHz bandwidth from a 660pF photodiode. Although this may not seem like much bandwidth, it necessitates a gain bandwidth product of at least 1.8GHz in the amplifier. The task of the LT1793 is to keep the JFET biased at its IDSS current (VGS = 0V); it was selected for its low 100pA maximum input offset current over temperature. The LT1793 senses the input voltage at the JFET gate through R1 and nulls this voltage through the LT1806 inverting pin and back around through R4. The time constants formed by R1-C1 and R3C3 ensure that the LT1793 noise characteristics do not add to the total noise. C1 shunts the already low LT1793 current noise to ground and R3-C3 keeps the LT1793 and resistor thermal noise away from the LT1806 www.BDTIC.com/Linear Linear Technology Magazine • May 2001 THEORETICAL 130nV/√Hz FLOOR continued on page 36 33 DESIGN IDEAS Conclusion sampling. Note that R3–R6 must be low TC resistors if gain stability is required. If the bridge has temperature characteristics that are more significant than the temperature coefficient of the resistive elements themselves, the effective value of R1 can be modified with a thermistor or, if the temperature is measured, via a DAC. Although this example uses 10V excitation and a 10V supply for the amplifier, the components have been chosen to allow 5V operation. If 5V operation is not a requirement, this approach can also be used with a lower noise amplifier such as the LT1124, which requires higher supply voltages. The LT1124 will not tolerate the capacitive load of C3 and C4, and these must be replaced with snubber circuits consisting of 33Ω and 1µF in series to ground. Consult the factory for recommendations on the use of other amplifiers in this circuit. The noise performance achievable with the LT1124 is on the order of 45nVRMS. The above example is just one of many ways that the LTC2411 can be used in a correlated double sampling scheme. This is a simple example but is capable of impressive results, although the implications of sensing the current through the bridge as opposed to the voltage across the bridge must be understood. Success is dependent on careful construction, although it is not as sensitive to temperature gradients as a conventional DC excitation scheme. R1 is very critical. The prototype in our lab can resolve 1 penny (2.5 grams) on a 350lb scale—in other words, 1 part in 140,000 on a load cell with a sensitivity of 1.5mV/V. Photodiode amplifier, continued from page 33 (1MΩ and 0.5pF) and the 75pF minimum input capacitance of the JFET, the gain of the circuit is 150 minimum above 300kHz. The LT1806 is a 325MHz gain bandwidth, unitygain-stable op amp, so it is quite comfortable maintaining stability above 300kHz in what it sees as a gain of about nineteen (150/8). Note that because the JFET circuit has a gain of six and the LT1806 has a gain of 400 at 1MHz, the gain bandwidth of the composite amplifier is nominally 2.4GHz3 . Also of interest are the open loop gains of 2.4 million (8 • 300,000) in the fast loop and 350 billion (3.5 million • 300,000/3) in the slow loop. These numbers, along with the gain bandwidth and the 1M feedback resistor, determine the impedance that the photodiode sees looking into the amplifier input. low noise op amp input. Note that with the JFET gate at 0V, there is no reverse bias across the photodiode, eliminating dark-current issues. At first glance, the circuit does not appear stable, since the JFET circuit puts additional gain into the op amp loop and this is usually a recipe for disaster. The reason the circuit is stable (and has quite a bit of margin) is that the gain is greater than unity at frequencies above a few hundred Hertz. Because of the relatively high value of the feedback impedances Table 1. Performance of composite amplifier with various photodiodes 500 OUTPUT NOISE DENSITY (nV/√Hz) E 400 A 300 D C 200 A B C 0 20k 60k 40k FREQUENCY (Hz) 80k Figure 3. Output spectra for various photodiodes 36 B 100 0 Notes: 1 Equivalent to Japanese 2SK147 2 For devices with IDSS higher than about 12mA, R5 should be reduced in value to avoid saturating the JFET. 3 A GBW of 2.4GHz was confirmed in the laboratory at f = 1MHz. 100k D E Optical Vendor Part Number Properties Siemens/Infineon SFH213 Fast IR PIN (408) 456-4071 Siemens/Infineon Enhanced Blue BPW34B (408) 456-4071 PIN Opto-Diode Narrow IR ODD45W (805) 499-0335 GaAIAs Fermionics Extended IR FD1500W (805) 582-0155 InGaAs Siemens/Infineon Visible BPW21 (408) 456-4071 Spectrum www.BDTIC.com/Linear Typical V = 0 Capacitance Approximate Bandwidth 11pF 250kHz 72pF 390kHz 170pF 380kHz 300pF 500kHz 660pF 650kHz Linear Technology Magazine • May 2001