V

OUT

TRIM

R2

R1

10k

1k

1.2V

REF5020-EP, REF5025-EP, REF5040-EP, REF5050-EP

SBOS471B – APRIL 2010 – REVISED JUNE 2015

www.ti.com

Typical Applications (continued)

Table 1. Noise Measurement Results for Different Load Capacitors

22 kHz

30 kHz

80 kHz

NOISE

>500 kHz

UNIT

LP-5P

LP-3P

LP-3P

GND

0.8

1

1.8

4.9

1 µF

37.8

41.7

53.7

9,017

2.2 µF (cer)

41.7

46.2

55.1

60.8

10 µF

33.4

33.4

35.2

38.5

10 µF (cer)

37.1

37.2

37.8

39.1

20 µF (cer)

33.1

33.1

33.2

34.5

47 µF

23.2

23.8

24.1

26.5

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-m

Ω) 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.

16

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