qf4a512 ETC-unknow, qf4a512 Datasheet - Page 17

• Manufacturer Part Number: qf4a512
• Description: 4-channel Programmable Signal Converter Psc
• Manufacturer: ETC-unknow

6. ANALOG FRONT END (AFE)
6.1 Configurable parameters
The following items are configurable for the AFE:
These settings can all be configured via the Quickfilter Pro software.
6.2 Chopper-stabilized Amplifier
Low frequency flicker noise, which is inherent to CMOS devices, limits maximum dynamic range that can be attained solely by
oversampling and averaging data from the analog front end (AFE). This flicker noise has a 1/f^n characteristic, which dominates other
noise sources at low frequencies of interest. This is in contrast to white noise, with a flat frequency spectrum, which proportionally
decreases with oversampling and averaging.
Chopper stabilization is a technique that applies modulation to transpose the signal of interest to a higher frequency where there is no
flicker noise, and then demodulate it back to the baseband after amplification. Details can be found in the following paper: ‘Circuit
Techniques for Reducing the Effects of Op-Amp Imperfections: Autozeroing, Correlated Double Sampling, and Chopper Stabilisation’
By Enz and Temes, Proceedings of the IEEE, November 1996.
In the QF4A512, the circuit is chopped at the input (prior to the PGA) and is remodulated back to the baseband after the analog to
digital conversion has occurred. Harmonic components of the chopper frequency are subsequently filtered out in the CIC and CIH
blocks. The chopper circuit improves the SNR for configurations with data rates 200 kHz and below. Two important settings in the
QF4A512 are the chopper frequency and the phase offset between the modulator and demodulator.
The chopper frequency is set with a divider whose setting is closely related to the CIC R decimation value. The chopper frequency is
derived from the ADC clock and the divider value is stored in the CHPC_DIVn register for each channel. The best setting for the
chopper divider is 2 x R x (# of active channels) in most cases. This setting puts the chopper 2nd harmonic frequency in the first null of
the CIC filter to most effectively remove it from the final digital signal. While the chopper circuit is effective at signal data rates of 200
kHz and below (down to Hz), the chopper signal itself is most effective at 10 kHz and above, which means the chopper divider should
track the CIC R value down only to the point where the chopper frequency would be 10 kHz. For example, for a 4 channel configuration,
with the ADC clock running at 50 MHz, and output data rates of 10 kHz and below, the chopper divider should be set no higher than
4E2h (decimal 1250) to ensure the chopper frequency is 10 kHz or above.
The chopper phase accounts for the group delay of the AAF and the pipeline delay of the ADC to ensure the modulated signal is
demodulated in the correct phase. The value, which ranges from 680-810 ns, is factory calibrated.
6.3 Programmable Gain Amplifier (PGA)
The PGA buffers and amplifies analog input signals prior to conversion and filtering. The input impedance of the PGA is around 10K on
both the positive and negative inputs. The PGA incorporates a chopper stabilized amplifier design to minimize dc offset and noise.
The PGA can be configured as either single-ended or differential and can receive input signals of up to 2.0V p-p directly. With the
addition of a single scaling resistor in each channel, two if configured differentially, the PGA can receive signals of up to +/- 10Vp-p or
higher. (see Section 14, Applications Circuits)
The PGA gain for each channel can be set to a value of 1, 2, 4 or 8, allowing for the optimum signal level to be passed on for further
processing. Usually this will be configured automatically by the Quickfilter Pro software, the resultant value can be verified in the
CHn_PGA register (n = channel number). The gain should be set as high as possible for best SNR, but not too high to cause clipping
of the input signal.
Rev C5, Jan 07
1.
2.
3.
4.
Chopper on or off
PGA Gain
AAF Cutoff Frequency
Channel on or off
PRELIMINARY
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QF4A512