Datasheet AD7811, AD7812 (Analog Devices) - 6

AD7811/AD7812 TERMINOLOGY
usually distanced in frequency from the original sine waves
Signal to (Noise + Distortion) Ratio
while the third order terms are usually at a frequency close to This is the measured ratio of signal to (noise + distortion) at the the input frequencies. As a result, the second and third order output of the A/D converter. The signal is the rms amplitude of terms are specified separately. The calculation of the inter- the fundamental. Noise is the rms sum of all nonfundamental modulation distortion is as per the THD specification where it is signals up to half the sampling frequency (fS/2), excluding dc. the ratio of the rms sum of the individual distortion products to The ratio is dependent upon the number of quantization levels the rms amplitude of the fundamental expressed in dBs. in the digitization process; the more levels, the smaller the
Channel-to-Channel Isolation
quantization noise. The theoretical signal to (noise + distor- Channel-to-channel isolation is a measure of the level of tion) ratio for an ideal N-bit converter with a sine wave input crosstalk between channels. It is measured by applying a full- is given by: scale 20 kHz sine wave signal to all nonselected input channels Signal to (Noise + Distortion) = (6.02N + 1.76) dB and determining how much that signal is attenuated in the selected Thus for a 10-bit converter, this is 62 dB. channel. The figure given is the worst case across all four or eight channels for the AD7811 and AD7812 respectively.
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of
Relative Accuracy
harmonics to the fundamental. For the AD7811 and AD7812 Relative accuracy, or endpoint nonlinearity, is the maximum it is defined as: deviation from a straight line passing through the endpoints of the ADC transfer function. V 2 +V 2 +V 2 +V 2 +V 2
Differential Nonlinearity
THD (dB) = 20 log 2 3 4 5 6 This is the difference between the measured and the ideal V1 1 LSB change between any two adjacent codes in the ADC. where V1 is the rms amplitude of the fundamental and V2, V3,
Offset Error
V4, V5 and V6 are the rms amplitudes of the second through the This is the deviation of the first code transition (0000 . 000) sixth harmonics. to (0000 . 001) from the ideal, i.e., AGND + 1 LSB.
Peak Harmonic or Spurious Noise Offset Error Match
Peak harmonic or spurious noise is defined as the ratio of the This is the difference in Offset Error between any two channels. rms value of the next largest component in the ADC output spectrum (up to f /2 and excluding dc) to the rms value of the
Gain Error
S fundamental. Normally, the value of this specification is This is the deviation of the last code transition (1111 . 110) determined by the largest harmonic in the spectrum, but for to (1111 . 111) from the ideal, i.e., V – 1 LSB, after the REF parts where the harmonics are buried in the noise floor, it will offset error has been adjusted out. be a noise peak.
Gain Error Match Intermodulation Distortion
This is the difference in Gain Error between any two channels. With inputs consisting of sine waves at two frequencies, fa and
Track/Hold Acquisition Time
fb, any active device with nonlinearities will create distortion Track/hold acquisition time is the time required for the output products at sum and difference frequencies of mfa ± nfb where of the track/hold amplifier to reach its final value, within m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for ± 1/2 LSB, after the end of conversion (the point at which the which neither m nor n are equal to zero. For example, the track/hold returns to track mode). It also applies to situations second order terms include (fa + fb) and (fa – fb), while the where a change in the selected input channel takes place or third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and where there is a step input change on the input voltage applied (fa – 2fb). to the selected V input of the AD7811 or AD7812. It means IN The AD7811 and AD7812 are tested using the CCIF standard that the user must wait for the duration of the track/hold acquisi- where two input frequencies near the top end of the input tion time after the end of conversion or after a channel change/ bandwidth are used. In this case, the second and third order step input change to V before starting another conversion, to IN terms are of different significance. The second order terms are ensure that the part operates to specification. –6– REV. B C Document Outline Features General Description Product Highlights Functional Block Diagram Specifications Timing Characteristics Absolute Maximum Ratings ESD Caution Pin Configurations Pin Function Descriptions Terminology Signal to (Noise + Distortion) Ratio Total Harmonic Distortion Peak Harmonic or Spurious Noise Intermodulation Distortion Channel-to-Channel Isolation Relative Accuracy Differential Nonlinearity Offset Error Offset Error Match Gain Error Gain Error Match Track/Hold Acquisition Time Control Register (AD7811) Control Register (AD7812) Circuit Description Converter Operation Typical Connection Diagram Analog Input DC Acquisition Time AC Acquisition Time On-Chip Reference ADC Transfer Function Power-Down Options Power-On-Reset Power-Up Times Mode 2 Full Power-Down (PD1 = 1, PD0 = 0) Mode 2 Partial Power-Down (PD1 = 0, PD0 = 1) Power vs. Throughput Operating Modes Mode 1 Operation (High Speed Sampling) Mode 2 Operation (Automatic Power-Down) Serial Interface Simplifying the Serial Interface Microprocessor Interfacing AD7811/AD7812 to PIC16C6x/7x AD7811/AD7812 to MC68HC11 AD7811/AD7812 to 8051