PLL_OUT
3rd Order
Modulator
ΣΔ
CH0_IN
CH1_IN
+
-
PGA
SDHSDT
DTC
System
Memory
Data Buffer
f
system
Window
Comparator
SDHS
Charge
Pump
Input MUX
SDHS Functional Operation
570
SLAU367P – October 2012 – Revised April 2020
Copyright © 2012–2020, Texas Instruments Incorporated
Sigma-Delta High Speed (SDHS)
Figure 22-2. SDHS Block Diagram
22.2.1 Input Multiplexer
The input multiplexer is mentioned for completeness here, but is part of the physical interface (PHY). See
the PHY module (the SAPH module) for details on how to select the input channels, how bias is provided,
and what other features are available on a given device.
22.2.2 Third-Order Modulator
The sigma-delta ADC consists of two parts: modulator and digital filter. A third-order modulator is used in
SDHS. The modulator performs oversampling up to 80 MHz against the analog input signal and feeds the
digital filter a pulse code modulated (PCM) data stream. The digital filter averages the bitstreams from the
modulator over a given number of bits and generates output data at a reduced sampling rate, specified by
the oversampling rate (OSR), for further data processing.
Averaging can be used to increase the signal-to-noise performance of a conversion (see
a
and b). With a conventional ADC, each factor-of-4 oversampling can improve the SNR by approximately
6 dB or 1 bit. To achieve a 16-bit resolution out of a simple 1-bit ADC would require an impractical
oversampling rate of 4
15
= 1 073 741 824. To overcome this limitation, the sigma-delta modulator
implements a technique called noise shaping. Using a feedback loop and integrators, the quantization
noise is pushed to higher frequencies and, thus, much lower oversampling rates are sufficient to achieve
high resolutions (see
c).