8.2.2.1.1 Load Capacitance

To determine how much noise the reference voltage is contributing in a real application, this design uses the circuit presented in Figure 28. For the same conditions as power supply, input decoupling, and load current, measure the output noise for different output decoupling or load capacitors. The load capacitor type will change the low-pass filter frequency that is created on the output. This filter is determined by an added capacitor value and two parasitic components: the open-loop output impedance of the internal amplifier to the reference voltage, and the ESR of the external capacitor.

Figure 29 shows a fast-Fourier-transform (FFT) plot of the output signal of the reference voltage circuit with a 10-μF ceramic capacitor load. The output noise level peaks at around 9 kHz because of the response of the internal amplifier of the circuit to the capacitive load (C_L).

This peaking is the main contributor to the overall measured noise. This output noise, measured with an analog meter over a frequency range of up to 80 kHz, is approximately 16.5 μVRMS. If the voltage-reference circuit was connected to the input of an ADC, the measured noise across a 65-kHz frequency range would be 138 μVPP. This noise level makes this solution adequate for 8- to 14-bit converters.

Figure 29. REF50xx FFT Plot of the Noise
With 10-µF Load Capacitor and 10-µΩ ESR

Every capacitor can be represented with a complicated equivalent model, which is voltage and frequency dependent with a large number of passive components. For the purposes of this design, this model is limited to the few components. The biggest impact on the creation of the low-pass filter and stability analysis is the simplified model of equivalent series inductance and resistance. Considering good layout practice and inherently low equivalent series inductance of today’s components, this model in the future analysis will be presented only by equivalent capacitance and series resistance.

Figure 30. Equivalent SCH Of REF50xx With Load Capacitor for Stability Analysis

When evaluating the impact of ESR and C_L on the performance the reference voltage, it is important to include the effect of the open-loop output resistance (R_O) of the output amplifier. The combination of R_O, ESR, and C_L modifies the open-loop response curve by introducing one pole (f_P) and one zero (f_Z). The values R_O, ESR, and C_L determine the corner frequency of the added pole f_P; and the values of ESR and C_L determine the corner frequency of the added zero.

The introduction of the external ESR-CL on the output of the reference voltage modifies the output amplifier open-loop gain curve. The added pole modifies the open-loop gain curve of the reference voltage output amplifier by introducing a –20 dB/decade change at the frequency f_P to the already –20 dB/decade slope of the open-loop gain curve, making the slope equal to –40 dB/decade. The added zero at frequency f_Z changes the open-loop gain curve back to –20 dB/decade.

Table 1 shows the measured noise values for different frequency bandwidths as well as different values and types of external capacitors. These measurements show that low-ESR (approximately 100-mΩ) ceramic capacitors tend to increase the noise, compared to normal-ESR (approximately 2-Ω) tantalum capacitors. This tendency is caused by a stability issue with the output amplifier and gain peaking in the amplifier frequency response.

8.2.2.1.2 Bandgap Noise Reduction

Figure 31. REF50xx Internal Structure of Trim/NR Pin

The internal schematic of the REF50xx device shows that the trim pin allows direct access to the bandgap output. Figure 31 shows the trim pin connection to the internal bandgap circuit through a resistor. Adding a capacitor on the trim pin creates a lowpass filter that has a broadband attenuation of −21 dB.

For example, a small 1-μF capacitor adds a pole at 14.5 Hz and a zero at 160 Hz. If more filtering is needed, a larger value capacitor can be added, which will lower the filter cutoff frequency and the noise contributed by the bandgap.

Adding a 1-μF capacitor in this example filters the noise contribution of the bandgap and lowers the total noise by a factor of 2.5 times.