Datasheet LTC1278 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | 12-Bit, 500ksps Sampling A/D Converter with Shutdown |
| Pages / Page | 16 / 9 — APPLICATIONS INFORMATION. Effective Number of Bits. DYNAMIC PERFORMANCE |
| File Format / Size | PDF / 290 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Effective Number of Bits. DYNAMIC PERFORMANCE
Model Line for this Datasheet
- LTC1278-4CN#PBF LTC1278-4CSW#PBF LTC1278-4CSW#TRPBF LTC1278-4IN#PBF LTC1278-4ISW#PBF LTC1278-4ISW#TRPBF LTC1278-5CSW#PBF LTC1278-5CSW#TRPBF LTC1278-5ISW#PBF LTC1278-5ISW#TRPBF
Text Version of Document
LTC1278
U U W U APPLICATIONS INFORMATION
compare mode. The input switch switches CSAMPLE to a 500kHz sampling rate and a 100kHz input. The dynamic ground, injecting the analog input charge onto the sum- performance is excellent for input frequencies up to the ming junction. This input charge is successively com- Nyquist limit of 250kHz. pared with the binary-weighted charges supplied by the capacitive DAC. Bit decisions are made by the high speed
Effective Number of Bits
comparator. At the end of a conversion, the DAC output The Effective Number of Bits (ENOBs) is a measurement of balances the AIN input charge. The SAR contents (a 12-bit the resolution of an ADC and is directly related to the data word) which represent the AIN are loaded into the S/(N + D) by the equation: 12-bit output latches. N = [S/(N + D) – 1.76]/6.02
DYNAMIC PERFORMANCE
where N is the Effective Number of Bits of resolution and The LTC1278 has excellent high speed sampling capabil- S/(N + D) is expressed in dB. At the maximum sampling ity. FFT (Fast Fourier Transform) test techniques are used rate of 500kHz the LTC1278 maintains very good ENOBs up to test the ADC’s frequency response, distortion and to the Nyquist input frequency of 250kHz. Refer to Figure 3. noise at the rated throughput. By applying a low distor- tion sine wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be exam- 12 74 ined for frequencies outside the fundamental. Figure 2 11 68 shows a typical LTC1278 FFT plot. 10 62 NYQUIST 9 FREQUENCY 56 8 S/(N 7 0 +D) (dB) fSAMPLE = 500kHz ±5V 6 fIN = 97.045kHz 5 –20 4 3 –40 EFFECTIVE NUMBER OF BITS 2 1 fSAMPLE = 500kHz –60 0 10k 100k 1M 2M AMPLITUDE (dB) –80 INPUT FREQUENCY (Hz) LT1278 G4 –100
Figure 3. Effective Bits and Signal-to-Noise + Distortion vs Input Frequency
–120 0 50k 100k 150k 200k 250k FREQUENCY (Hz) LTC1278 F2
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the RMS
Figure 2. LTC1278 Nonaveraged, 4096 Point FFT Plot
sum of all harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is
Signal-to-Noise Ratio
expressed as: The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the √V 2 + V 2 + V 2 ... + V 2 ratio between the RMS amplitude of the fundamental input THD = 20log 2 3 4 N V frequency to the RMS amplitude of all other frequency 1 components at the A/D output. The output is band limited where V1 is the RMS amplitude of the fundamental fre- to frequencies from above DC and below half the sampling quency and V2 through VN are the amplitudes of the frequency. Figure 2 shows a typical spectral content with second through Nth harmonics. THD versus input 9
U U W U APPLICATIONS INFORMATION
compare mode. The input switch switches CSAMPLE to a 500kHz sampling rate and a 100kHz input. The dynamic ground, injecting the analog input charge onto the sum- performance is excellent for input frequencies up to the ming junction. This input charge is successively com- Nyquist limit of 250kHz. pared with the binary-weighted charges supplied by the capacitive DAC. Bit decisions are made by the high speed
Effective Number of Bits
comparator. At the end of a conversion, the DAC output The Effective Number of Bits (ENOBs) is a measurement of balances the AIN input charge. The SAR contents (a 12-bit the resolution of an ADC and is directly related to the data word) which represent the AIN are loaded into the S/(N + D) by the equation: 12-bit output latches. N = [S/(N + D) – 1.76]/6.02
DYNAMIC PERFORMANCE
where N is the Effective Number of Bits of resolution and The LTC1278 has excellent high speed sampling capabil- S/(N + D) is expressed in dB. At the maximum sampling ity. FFT (Fast Fourier Transform) test techniques are used rate of 500kHz the LTC1278 maintains very good ENOBs up to test the ADC’s frequency response, distortion and to the Nyquist input frequency of 250kHz. Refer to Figure 3. noise at the rated throughput. By applying a low distor- tion sine wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be exam- 12 74 ined for frequencies outside the fundamental. Figure 2 11 68 shows a typical LTC1278 FFT plot. 10 62 NYQUIST 9 FREQUENCY 56 8 S/(N 7 0 +D) (dB) fSAMPLE = 500kHz ±5V 6 fIN = 97.045kHz 5 –20 4 3 –40 EFFECTIVE NUMBER OF BITS 2 1 fSAMPLE = 500kHz –60 0 10k 100k 1M 2M AMPLITUDE (dB) –80 INPUT FREQUENCY (Hz) LT1278 G4 –100
Figure 3. Effective Bits and Signal-to-Noise + Distortion vs Input Frequency
–120 0 50k 100k 150k 200k 250k FREQUENCY (Hz) LTC1278 F2
Total Harmonic Distortion
Total Harmonic Distortion (THD) is the ratio of the RMS
Figure 2. LTC1278 Nonaveraged, 4096 Point FFT Plot
sum of all harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is
Signal-to-Noise Ratio
expressed as: The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the √V 2 + V 2 + V 2 ... + V 2 ratio between the RMS amplitude of the fundamental input THD = 20log 2 3 4 N V frequency to the RMS amplitude of all other frequency 1 components at the A/D output. The output is band limited where V1 is the RMS amplitude of the fundamental fre- to frequencies from above DC and below half the sampling quency and V2 through VN are the amplitudes of the frequency. Figure 2 shows a typical spectral content with second through Nth harmonics. THD versus input 9
