MCP3919B datasheet(29/81 Pages) MICROCHIP

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DS20006995A-page 29

MCP3919B

5.0

DEVICE OVERVIEW

5.1

Analog Inputs (CHn+/-)

The MCP3919B analog inputs can be connected

directly to current and voltage transducers (such as

shunts, Current Transformers, or Rogowski coils).

Each input pin is protected by specialized ESD

structures that allow bipolar ±2V continuous voltage,

with respect to AGND, to be present at their inputs

without the risk of permanent damage.

All channels have fully differential voltage inputs for

better noise performance. Maintain the absolute volt-

age at each pin, relative to AGND, in the ±1V range

during operation to ensure the specified ADC accuracy.

Adapt the Common-mode signals to respect both the

previous conditions and the differential input voltage

range. For best performance, maintain the Common-

mode signals to AGND.

5.2

Programmable Gain Amplifiers

(PGA)

The three Programmable Gain Amplifiers (PGAs)

reside at the front end of each Delta-Sigma ADC. They

have two functions: translate the Common-mode volt-

age of the input from AGND to an internal level between

AGND and AVDD, and amplify the input differential

signal. The translation of the Common-mode voltage

does not change the differential signal, but recenters

the Common-mode so that the input signal can be

properly amplified.

The PGA block can be used to amplify very low signals,

but the differential input range of the Delta-Sigma

modulator must not be exceeded. The PGA on each

channel is independent and is controlled by the

PGA_CHn[2:0] bits in the GAIN register. Table 5-1

displays the gain settings for the PGA.

5.3

Delta-Sigma Modulator

5.3.1

ARCHITECTURE

All ADCs are identical in the MCP3919B and they

include a proprietary second-order modulator with a

multibit 5-level DAC architecture (see Figure 5-1). The

quantizer is a Flash ADC composed of four compara-

tors with equally spaced thresholds and a thermometer

output coding. The proprietary 5-level architecture

ensures minimum quantization noise at the outputs of

the modulators without disturbing linearity or inducing

additional distortion. The sampling frequency is

DMCLK (typically 1 MHz with MCLK = 4 MHz), so the

modulators are refreshed at a DMCLK rate.

Figure 5-1 represents a simplified block diagram of the

Delta-Sigma ADC present on the MCP3919B.

FIGURE 5-1:

Simplified Delta-Sigma ADC

Block Diagram.

Note:

If the analog inputs are held to a potential

of -0.6 to -1V for extended periods of time,

MCLK must be present inside the device

in order to avoid large leakage currents at

the analog inputs. However, during the

Shutdown mode of all the ADCs or POR

state, the clock is not distributed inside the

circuit. During these states, it is recom-

mended to keep the analog input voltages

above

-0.6V, referred to as AGND, to

avoid high analog input leakage currents.

TABLE 5-1:

PGA CONFIGURATION

SETTING

GAIN

PGA_CHn[2:0]

Gain

(V/V)

Gain

(dB)

VIN = (CHn+) – (CHn-)

Differential

Input Range (V)

±0.6

±0.3

±0.15

±0.075

±0.0375

±0.01875

Note:

The two undefined settings are G = 1. This table

is defined with VREF = 1.2V.

Second-

Order

Integrator

Loop

Filter

Quantizer

DAC

Differential

Voltage Input

Output

Bitstream

5-Level

Flash ADC

MCP3919B Delta-Sigma Modulator

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