ZHCSCX2E January 2014 – October 2017 LM15851
PRODUCTION DATA.
To ensure that system-gain management has the quickest-possible response time, a low-latency configurable over-range function is included. The over-range function works by monitoring the raw 12-bit samples exiting the ADC module. The upper 8 bits of the magnitude of the ADC data are checked against two programmable thresholds, OVR_T0 and OVR_T1. The following table lists how a raw ADC value is converted to an absolute value for a comparison of the thresholds.
ADC SAMPLE (OFFSET BINARY) | ADC SAMPLE (2's COMPLEMENT) | ABSOLUTE VALUE | UPPER 8 BITS USED FOR COMPARISON |
---|---|---|---|
1111 1111 1111 (4095) | 0111 1111 1111 (+2047) | 111 1111 1111 (2047) | 1111 1111 (255) |
1111 1111 0000 (4080) | 0111 1111 0000 (+2032) | 111 1111 0000 (2032) | 1111 1110 (254) |
1000 0000 0000 (2048) | 0000 0000 0000 (0) | 000 0000 0000 (0) | 0000 0000 (0) |
0000 0001 0000 (16) | 1000 0001 0000 (-2032) | 111 1111 0000 (2032) | 1111 1110 (254) |
0000 0000 0000 (0) | 1000 0000 0000 (-2048) | 111 1111 1111 (2047) | 1111 1111 (255) |
If the upper 8 bits of the absolute value equal or exceed the OVR_T0 or OVR_T1 threshold during the monitoring period, then the over-range bit associated with the threshold is set to 1, otherwise the over-range bit is 0. The resulting over-range bits are embedded into the complex output data samples and output on OR_T0 and OR_T1. Table 3 lists the outputs, related data samples, threshold settings and the monitoring period equation.
EMBEDDED OVER-RANGE OUTPUTS | ASSOCIATED THRESHOLD | ASSOCIATED SAMPLES | MONITORING PERIOD (ADC SAMPLES) |
---|---|---|---|
OR_T0 | OVR_T0 | In-Phase (I) samples | 2OVR_N(1) |
OR_T1 | OVR_T1 | Quadrature (Q) samples |
OVR_N | MONITORING PERIOD |
---|---|
0 | 1 |
1 | 2 |
2 | 4 |
3 | 8 |
4 | 16 |
5 | 32 |
6 | 64 |
7 | 128 |
Typically, the OVR_T0 threshold can be set near the full-scale value (228 for example). When the threshold is triggered, a typical system can turn down the system gain to avoid clipping. The OVR_T1 threshold can be set much lower. For example, the OVR_T1 threshold can be set to 64 (−12 dBFS). If the input signal is strong, the OVR_T1 threshold is tripped occasionally. If the input is quite weak, the threshold is never tripped. The downstream logic device monitors the OVR_T1 bit. If OVR_T1 stays low for an extended period of time, then the system gain can be increased until the threshold is occasionally tripped (meaning the peak level of the signal is above −12 dBFS).
The OR_T0 threshold is embedded as the LSB along with the upper 15 bits of every complex I sample. The OR_T1 threshold is embedded as the LSB along with the upper 15 bits of every complex Q sample.