When you choose an ADC for your design, you typically choose it for its resolution, depending on the system’s need for accuracy. Sometimes you may decide to go with a higher resolution converter (say 14-bit instead of 12-bit) just to have a “safety” margin. After choosing your ADC, you select the amplifier based on a number of parameters, with output swing being one of the most important. This is especially true in a single-supply operation with low voltage of 3V to 5V, and dynamic range not as wide as a higher voltage split-supply system. Even when your rail-to-rail input and output (RRIO) amplifier has a single-supply voltage from 0V to 5V, its output never quite reaches zero. It is limited by the output saturation (also known as dropout voltage or swing) regardless of the process or the topology of the amplifier.
So what impact does a limited output swing have on the ADC in this case? It prevents the full scale voltage from being processed in its entirety. For example, if an amplifier’s output swing low is 50 mV, and it’s driving a 12-bit ADC with a 2.5V reference, roughly the lowest 80 codes of the ADC’s input are lost (0.050V)/(2.50V/4096) = 81.9.
While this sometimes may be acceptable, as the load gets heavier, the dropout voltage becomes larger and the swing becomes a concern. For a 200mV output with the same conditions as above, 8% of the ADC’s input range is lost.
Even more of a concern, is having two-stage amplification as shown in Figure 1. The gain of the second amplifier (A2) is equal to: 1+RF2/RG2. The output of A1 (VOUT1) is amplified by the noise gain of A2.
If the input signal of A2 is 0V, while its gain is 5 and the output saturation voltage of A1 is 50mV, then the minimum output voltage of A2 is 250mV (50mV x 5 = 250mV). This means that the output of A2 will not swing below 250mV even if it is specified to 50mV.
Using the previous equation, you quickly determine that in this case, you now lose close to 410 bits, or about 10% of your 12-bit ADC.
Figure 1: Two-stage amplification can lose up to 10% of a 12-bit ADC’s resolution.
So how do you get a true zero and the output?
The LM7705 low noise negative bias generator is designed to work with virtually any single-supply op amp. It is optimized for a 5V single-supply design and provides the user with an external filter to minimize the output ripple caused by the charge pump. The LM7705 has an output of -0.23mV to ensure proper operation with any 5V op amp without exceeding the maximum absolute rating.
Figure 2 and Figure 3 show oscilloscope images highlighting the LMP7731 low-noise, RRIO, low-voltage amplifier and the LMC7101 low-power op amp with RRIO, respectively. Figure 2 clearly shows the output well above zero while Figure 3 shows a full swing using the LM7705 with the same op amps and under the same conditions.
| Figure 3: Scope shot of LMP7731 and LMC7101 swinging
to zero output using the LM7705.