New Amplifier features Low Noise Rail-to-Rail Performance While Consuming Only two.5mA
The new LT 6202 op amp combines 100MHz gainbandwidth, 1.9nV/√Hz voltage noise and rail-to-rail inputs and outputs, while consuming only 2.5mA. It also features a low 0.75pA/√Hz current noise, and it contributes exceptionally low total noise and distortion power in small-signal applications. The unit is fully specifi ed on 3V, 5V and ±5V supplies and is obtainable in commercial and industrial temperature grades both in the SOT and SO packages. Dual and quad versions can also be found as the LT6203 and LT6204, respectively.
Low Power, 2.4nV/√Hz, Photodiode AC Transimpedance Amplifier Outperforms Monolithic Solutions
You can't optimize for everything. Op amp designs that try to squeeze good JFETs to their high speed monolithic processes inevitably compromise other parameters, usually resulting in high supply currents. Figure 1 shows an easy method of getting the very best of all possible worlds while using LT6202 along with a low noise discrete JFET. The JFET acts as a source follower, buffering the inverting input from the LT6202 and which makes it ideal for our prime impedance feedback elements R1 and R2.
The LT6202 forces the JFET source to 0V, with R3 ensuring that the JFET runs an IDRAIN of 1mA. Because the JFET operates well below its minimum IDSS and it has a narrow selection of pinchoff voltages, the circuit is guaranteed to self bias just beneath ground, typically at about -0.5V. With no photocurrent signal within the photodiode, the LT6202 output sits in the same voltage and tracks it. When the photodiode is illuminated, the current must range from LT6202 output through R1 and R2 therefore the output goes up because it would with a normal transimpedance amplifier.
Amplifier input noise density and gain-bandwidth product were measured to become 2.4nV/√ Hz and 100MHz, respectively, with simply 3.8mA supply current. This really is unparalleled within the monolithic world where Five to six times the availability current could be expected for similar performance. The 100MHz gain-bandwidth product of the LT6202 is maintained in this circuit because the JFET includes a high gm, approximately 1/80Ω, which checks 4.99kΩ so its loop attenuation is less than 2%.
Total circuit input capacitance including board parasitics is measured at 3.5pF. This really is under the specifi ed CGS from the JFET since the JFET source isn't grounded but rather checks R3 and the high impedance op amp input. This fact combined with low input voltage noise makes the circuit well suited to both large and small photodetectors. Utilizing a small photodiode with 2.5pF junction capacitance and adjusting parasitic feedback capacitance for 4% overshoot within the transient response, closed loop bandwidth is 1.6MHz.
Figure 2 shows the LT6202 used in a way very similar to that shown in Figure 1. In this instance however, the JFET is not allowed to dictate the DC bias conditions. Rather than simply grounding the LT6202 non-inverting input, an LTC2050 drives it (and then the source) exactly to where it needs to be for zero JFET gate voltage. Adding the LTC2050 boosts the total supply current by about 1mA. AC performance is nearly just like the uncorrected circuit of Figure 1, using the additional benefi t of fine DC performance. Output offset is 200μV and output noise is 2mVP-P measured in a 20MHz bandwidth.
Single Supply 16-Bit ADC Driver
Figure 3 shows the LT6203 driving an LTC1864 unipolar 16-bit, 250ksps A/D converter. The bottom 1 / 2 of the LT6203 is within an increase of 1 and buffers the 0V negative full-scale signal VLOW in to the negative input of the LTC1864. The upper 1 / 2 of the LT6203 is in an increase of 10, referenced to the buffered voltage VLOW and drives the positive input of the LTC1864. The input range of the LTC1864 is 0V to 5V so for the best results the input range of VIN is from VLOW, about 0.4V to about 0.82V. Figure 4 shows an FFT obtained with a 10.1318kHz (coherent) input waveform, without any windowing or averaging. Spurious free dynamic range is seen to be 100dB.
Although the LTC1864 has a sample rate far below the gain bandwidth from the LT6203, by using this amplifier is not necessarily a case of overkill. A/D converters have sample apertures which are extremely narrow (infinitesimal so far as mathematicians are concerned) making demands on upstream circuitry far in excess of what the innocent looking sample rate would imply. Additionally, when an A/D converter requires a sample, it applies a small capacitor to its inputs causing a fair quantity of glitch energy and expects the voltage on the capacitor to settle towards the true value very quickly. Finally, the LTC1864 has a 20MHz analog input bandwidth and can be utilized in under sampling applications, again requiring a resource bandwidth and settling speed higher than the Nyquist criterion would imply.
The LT6202, LT6203 and LT6204 are fast, low noise amplifiers which have been optimized for low power consumption. Their rail-to-rail inputs and outputs provide fl exibility and simplicity of use and maximize dynamic range.