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AS8515

Datasheet - AS8515 Bottom Die Overvi ew

8 AS8515 Bottom Die Overview

The AS8515

Bottom die consists of two independent high resolution 16-bit SD analog to digital conversion channels. The measurement path

of these two channels integrates a programmable gain amplifier, chopper and de-chopper, sigma-delta modulator, decimator and a digital filter

for simultaneous measurement of Current and Voltage/Temperature.

The two measurement channels, namely the Current and Voltage/Temperature measurement channels have identical data path.

The input signal is amplified in the Programmable Gain Amplifier (PGA) with any of the selected gains of 1, 5, 25, 40 and 100 facilitating

measurement of a wide range of Current, voltage and temperature levels. Gain Settings for different input ranges and any associated restrictions

are explained in the Table 30.

Offset in the measurement path is minimized with the use of a chopper and a de-chopper at appropriate stages in the data path. By default the

chopper/de-chopper is ON in the measurement path. It may be disabled by programming the appropriate register.

The amplified input signal is converted into a single-bit pulse-density modulated stream by the Σ-Δ Modulator. A decimator acting as a low-pass

filter filters out the quantization noise and generates 16-bit data corresponding to the input signal. The decimation ratios of 64, 128 may be

selected in the first filter stage. For reducing data rate further, the second stage decimation can be used.

An optional FIR Filter is provided to offer matched low pass filter response typically required in lead acid battery sensor systems.

8.1 Current Measurement Channel

The voltage across a Shunt Resistor, connected in series with the Battery negative terminal, forms the input signal to the Current Measurement

channel. RSHH and RSHL are the Current measurement input pins. Offset in the input signal is nullified with the use of a chopper and a de-

chopper at appropriate stages in the data path. The programmable gain amplifier in the data path with programmable settings of 1, 5, 25, 40 and

100 enables measurement of current ranges from ±1A to ±1500A on a 100µ

Ω shunt. The sampled input signal is converted into a single-bit

pulse-density modulated stream by the Σ-Δ Modulator. A decimator acting as a low-pass filter filters out the quantization noise and generates 16-

bit data equivalent to the input current signal. The programmable input sampling rate and the decimation ratio determine the output data rates.

The data path can be programmed to provide sub 1Hz to 4kHz rates in the various modes available. An optional FIR filter specifically designed

for 1KHz sample rate is provided to offer matched low pass filter response typically required in lead acid battery sensor systems.

After enabling the current measurement channel, the delay for the availability of the first sample is two conversion cycles.

8.2 Voltage/Temperature Measurement Channel

The other two parameters of the Battery for measurement are Voltage and its Temperature. The second channel accepts signals from four

independent sources through a Multiplexer as listed below:



An attenuator battery voltage obtained through internal resistor divider from

Top die, (or)



A signal from the external temperature sensor, (or)



A signal from external reference, (or)



A signal from the internal temperature sensor.

Apart from this difference in the multiplexing of four input signals, the rest of the data path is identical to the Current measurement channel.

RSHH and RSHL are the Current measurement input pins.

The Battery Voltage which can go up to 18V is attenuated through a Resistor Divider externally and is applied to the Voltage Channel. For

Automotive Battery measurement, the PGA is to be bypassed to connect battery voltage attenuated by a factor of 21 directly to the ADC input.

The latency for the first result from the voltage measurement channel is two conversion cycles.

A second option on this measurement channel is to measure Temperature. Internally generated constant current is pumped through the

Temperature Sensor with positive temperature coefficient, and, a high- precision resistor. The voltages across the sensor and the resistor form

the inputs to the measurement channel one at a time. The difference between the two voltages which is independent of the magnitude of the

current is used to determine the temperature accurately. The voltage across the sensor is applied between the ETS and VSS pins and, the

voltage across the high-precision resistor is applied between ETR and VSS. External temperature measurement involves the acquisition of two

signals one after the other using the same constant current source. The latency for the first result from the temperature measurement channel is

two conversion cycles.

A third option on the measurement channel is to measure the internal temperature. Hence, one of the three options for measurement of Battery

Voltage, External Temperature and, internal temperature may be carried out by selection of appropriate inputs through the internal multiplexer

selection.

ETR and ETS inputs can optionally be used to measure other signal sources like external resistive attenuators for battery voltages different to

12V nominal.

ETR and ETS are single ended inputs and referenced to AVSS. Voltage drop on internal bond wire causes ~100 digits of offset with systematic

temperature dependency of another 50 LSB’s over temperature.

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