MCP3910 datasheet(31/90 Pages) MICROCHIP

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DS20005116D-page 31

MCP3910

5.0

DEVICE OVERVIEW

5.1

Analog Inputs (CHn+/CHn-)

The MCP3910 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. The absolute voltage at each

pin, relative to AGND, should be maintained in the ±1V

range during operation in order to ensure the specified

ADC accuracy. The Common-mode signals should be

adapted to respect both the previous conditions and

the differential input voltage range. For best

performance, the Common-mode signals should be

maintained to AGND.

5.2

Programmable Gain Amplifiers

(PGAs)

The Programmable Gain Amplifiers (PGAs) reside at

the front end of each Delta-Sigma ADC. They have two

functions: translate the Common-mode voltage 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 re-centers 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 for

Channel n 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 MCP3910 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 MCP3910.

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. This is true even during

Hard Reset mode or during the Soft Reset

of all ADCs. However, during the Shut-

down mode of all the ADCs or during the

POR state, the clock is not distributed

inside the circuit. During these states, it is

recommended 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 Range (V)

±0.6

±0.3

412

±0.15

818

±0.075

±0.0375

±0.01875

Note:

The two undefined settings are G = 1. This

table is defined with VREF =1.2V.

Loop

Filter

Quantizer

DAC

Differential

Voltage Input

Output

Bitstream

5-Level

Flash ADC

MCP3910 Delta-Sigma Modulator

Second-

Order

Integrator

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