Dynamic Range and SNR in Low Single Supply Systems
Reducing the operating supply voltage from a ±15-V split supply to a single 5-V supply significantly reduces the maximum available dynamic range. The dynamic range at the output is determined by the ratio of the largest output voltage to the smallest output voltage. An industry standard operational amplifier like the TLC271 is specified at 5-V single supply with 3.8 Vpp for the maximum output swing. This means that the whole supply span can not be used for the output swing, resulting in a further reduction of the maximum available dynamic range and SNR. A rail-to-rail operational amplifier like the TLV24xx family can use the full span of the supply range for signal conditioning at the input and output. Operational amplifier disturbance levels are independent of the supply voltage. This results in smaller spacing between usable and noise signals. If the operational amplifier is used with ac signals, by decoupling the signals from dc, then noise forms the determining disturbance signal. For a standard operational amplifier such as the TLC271C, the input noise voltage Vn at a signal bandwidth of 1 MHz equals 68 umV†= 68 nV/(Hz)^1/2*(1 MHz)^1/2. With a 5-V single supply, the reduced output range allows a maximum signal level of 3.8 Vpp. This results in a unity gain configuration in a SNR of 95.4 dB=20 log(4 V/68 uV). In the same configuration, a rail-to-rail amplifier such as the TLV246xI with Vn=11 nV/(Hz) )^1/2 † and a maximum signal level of 5 Vpp at the input and output provides a signal-to-noise ratio of 113 dB=20 log(5 V/11 umV) at BW=1 MHz. In a precision system the operational amplifier must amplify the dc voltage level precisely. Errors in this area result from offset and gain problems. In a 5-V system with a constant common-mode voltage, the TLC271C has an input offset voltage VIO of 1.1 uV†. This alone limits the dynamic range to 71 dB=20 log(3800/1.1) in a unity gain configuration. The TLV245x, however, with VIO = 20 mV† and the rail-to-rail characteristic has a significantly higher dynamic range of 108 dB=20 log(5000/0.02) in the same circuitry.
When signal-to-noise ratio and dynamic range are critical design parameters, rail-to-rail characteristics of the operational amplifier must ensure that these parameters are met.
Construction of a Rail-to-Rail Output Stage
useable with low load. Figure 3 shows this on the output level of the TLV243x and TLV246x.
Reduction of the output signal due to the load also results in a reduction of the open-loop gain AVD. Because the open-loop gain is dependent on the connected load, the load should always be considered during comparison of the open-loop gain of different amplifiers. Figure 4 shows the influence of a resistive load on the amplification of a TLV246x.
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