SBAS943A September 2018 – August 2019 ADS1284
PRODUCTION DATA.
A diagram of the input multiplexer is shown in Figure 33.
ESD diodes protect the multiplexer inputs. If either input is taken below AVSS – 0.3 V, or above AVDD + 0.3 V, the ESD protection diodes can turn on. If these conditions are possible, use external clamp diodes, series resistors, or both to limit the input current to safe values (see the Absolute Maximum Ratings table).
Overdriving one unused input can affect the conversions of the other input. If an overdriven input interacts with the measured input, clamp the overdriven signal with external Schottky diodes.
The specified input operating range of the PGA is shown in Equation 2:
For best operation, maintain absolute input levels (input signal level and common-mode level) within these limits.
The multiplexer connects one of the two external differential inputs to the preamplifier inputs, in addition to internal connections for various self-test modes. Table 5 summarizes the multiplexer configurations for Figure 33.
MUX[2:0] | SWITCHES | DESCRIPTION |
---|---|---|
000 | S1, S5 | AINP1 and AINN1 connected to preamplifier |
001 | S2, S6 | AINP2 and AINN2 connected to preamplifier |
010 | S3, S4 | Preamplifier inputs shorted together through 400-Ω internal resistors |
011 | S1, S5, S2, S6 | AINP1, AINN1 and AINP2, AINN2 connected together and to the preamplifier |
100 | S6, S7 | External short, preamplifier inputs shorted to AINN2 (common-mode test) |
The typical value of multiplexer on-resistance is 30 Ω (each switch). When the multiplexer is used to drive an external load connected to one channel by a signal generator connected to the other channel, on-resistance and on-resistance variation can lead to measurement errors. Figure 34 shows THD versus load resistance and amplitude (PGA gain). In this configuration, THD performance improves when used with high-impedance loads and low amplitude drive signals. The data are measured with the circuit from Figure 35 with the channel connected to each other for measurement (MUX[2:0] = 011).