Rainbow Electronics MAXQ7667 - Manual (Page 18)

MAXQ7667
16-Bit, RISC, Microcontroller-Based,
Ultrasonic Distance-Measuring System
18 ______________________________________________________________________________________
Sigma-Delta ADC
The MAXQ7667 features a 16-bit sigma-delta ADC with
an analog gain adjustable from 38dB to 60dB (includ-
ing the fixed LNA gain) with a maximum gain step of
12.5% (typical). Gain changes settle within one ADC
conversion. Use software to create a virtual time vari-
able gain amplifier. A digital bandpass and lowpass fil-
ters remove switching glitches and DC offset at the
output of the ADC.
In a typical application, the software sets the gain to a
low value when the burst is first sent and increases the
gain as the time from when the burst was sent increas-
es. As a result, strong echoes from nearby objects are
processed without clipping while small signals from dis-
tant objects are processed with the maximum gain. The
ADC samples the amplified echo signal from the LNA at
80 times the burst output frequency. The ADC provides
conversion results at a data rate equal to 10 times the
burst output frequency. The ADC conversion results
also load to an 8-deep first-in-first-out (FIFO) at the
native data rate or a separate time base without loading
the CPU.
Digital Bandpass Filter
The digital bandpass filter has a center frequency that
tracks the burst output frequency. The bandpass width
is 14% of the center frequency. The bandpass filter pro-
vides the 16-bit output data at a data rate equal to 10
times the burst output frequency.
Full-Wave Rectifier
The full-wave rectifier detects the envelope of the digital
bandwidth filter output to generate a DC output propor-
tional to the peak-to-peak amplitude of the input signal.
Full-wave rectification allows the digital lowpass filter to
respond faster without excessive ripple.
Digital Lowpass Filter
The lowpass filter removes the ripple from the full-wave
detector output. The output of the lowpass filter is avail-
able at a data rate equal to five times the burst output
frequency. The corner frequency is 1/5 the burst fre-
quency with approximately 40dB per decade rolloff.
The 16-bit output data of the lowpass filter is stored in a
FIFO register with a depth of eight samples. The
MAXQ7667 allows data transfer from the lowpass filter
SARC[2:0]:SARS[2:0]
SARC[11:9]:SARMX[2:0]
REFERENCE TO
SIGMA-DELTA ADC
ADC DATA
READY INTERRUPT
ASR.0:SARRDY
APE.12:BGE
APE.14:RBUFE
AIE.0:SARIE
BANDGAP
REF
SARC.3:SARBY
SARC.6:SARDUL
SARC.4:SARASD
SARC.7:SARBIP
SARC.8:SARDIF
OSCC[3:2]:SARCD[1:0]
ADC
CLOCK
DIV
TIMER 0
TIMER 1
TIMER 2
APE.4:SARE ADCCLK
AVDD
V REF
MUX
SARC[11:9]:SARMX[2:0]
12-BIT
ADC
AIN0
MUX
AIN1
AIN2
AIN3
DATA BUS[15:0]
AIN4
V REF
AGND
AVDD
AGND
REFBG
ADCCTL
REF
SYSCLK
BUF
x1.0
Figure 3. SAR ADC Block Diagram