CN-0385

Circuit Note

Rev. 0 | Page 8 of 13

Table 5 shows the gain error measured for each of the gain

configurations for the circuit in Figure 1. The %FS error was

found using the analysis methods described previously, and the

actual gain in V/V was calculated by subtracting this error from

the ideal gain.

Table 5. Gain Error Measurements for all Gain Configurations

Gain

Gain Error (%FS)

0.4

0.02

0.8

0.02

1.6

0.03

3.2

0.02

Performance Results Without Channel Switching

Figure 10, Figure 11, Figure 12, and Figure 13 show the fast

Fourier transform (FFT) plots for a 10 kHz, full-scale, sine wave

input on a single channel for gain configurations of 0.4, 0.8, 1.6,

and 3.2, respectively. Table 6 shows the SNR and rms noise

measured for each of the gain configurations.

Table 6. SNR, Noise, and THD vs. Gain for 10 kHz Input

Gain

SNR (dB)

RMS Noise (µV rms)

THD (dB)

0.4

93.9

55.2

−99.2

0.8

92.8

62.6

−98.5

1.6

90.6

80.7

−97.0

3.2

88.0

108.9

−94.6

The input signal was supplied by an Audio Precision SYS-2700

series signal generator, with the board set in differential input

mode. Figure 14 shows total harmonic distortion (THD)

measurements vs. the frequency of the input signal for each

gain configuration.

Figure 10. FFT for 10 kHz, 20 V p-p Input for Gain = 0.4 on Single,

Static Channel

Figure 11. FFT for 10 kHz, 10 V p-p Input for Gain = 0.8 on Single,

Static Channel

Figure 12. FFT for 10 kHz, 5 V p-p Input for Gain = 1.6 on Single,

Static Channel

Figure 13. FFT for 10 kHz, 2.5 V p-p Input for Gain = 3.2 on Single,

Static Channel

–200

–180

–160

–140

–120

–100

–80

–60

–40

–20

0

0

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

FREQUENCY (kHz)

SNR = 92.52dB

THD = –93.96dB

–200

–180

–160

–140

–120

–100

–80

–60

–40

–20

0

0

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

FREQUENCY (kHz)

SNR = 91.48dB

THD = –94.35dB

SINAD = 89.67dB

–200

–180

–160

–140

–120

–100

–80

–60

–40

–20

0

0

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

FREQUENCY (kHz)

SNR = 90.25dB

THD = –94.12dB

SINAD = 88.76dB

–200

–180

–160

–140

–120

–100

–80

–60

–40

–20

0

0

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

FREQUENCY (kHz)

SNR = 87.77dB

THD = –93.27dB

SINAD = 86.69dB