AD5940 datasheet(49/134 Pages) AD

49 / 134 page

Data Sheet

AD5940/AD5941

Rev. C | Page 49 of 134

HIGH SPEED TIA CIRCUITS

The high speed TIA measures wide bandwidth input signals up

to 200 kHz.

The output of the high speed TIA is connected to the main ADC

mux, where this output can be programmed as the ADC input

channel.

This block is designed for impedance measurements in

conjunction with the high speed DAC and excitation amplifier.

HIGH SPEED TIA CONFIGURATION

The high speed TIA is disabled by default and is turned on by

setting AFECON [11] = 1. The high speed TIA has programmable

flexibility built into the input signal selection, gain resistor

selection, input load resistor selection, and common-mode

voltage source.

Input Signal Selection

The input signal options are as follows:

•

The SE0 input pin.

•

The AIN0, AIN1, AIN2, and AIN3/BUF_VREF1V8

input pins.

•

The DE0 input pin, which has its own RLOAD/RTIA options

and is user programmable.

Gain Resistor Selection

The gain resistor (RTIA) options are 50 Ω to 160 kΩ for the DE0

input, and 200 Ω to 160 kΩ for all other input pins.

Load Resistor Selection

The load resistor (RLOAD) options are as follows:

•

RLOAD02 and RLOAD04 are fixed 100 Ω for SE0 and AFE3.

•

For the DE0 pin, RLOAD is programmable. The user can

select values from 0 Ω, 10 Ω, 30 Ω, 50 Ω, and 100 Ω.

Common-Mode Voltage Selection

The high speed TIA common-mode voltage setting, on the positive

input to the high speed TIA amplifier, is configurable. The

configuration options are as follows:

•

Internal 1.11 V reference source, which is the same as the

VBIAS_CAP pin voltage.

•

Low power DAC output (VZERO0).

Figure 29 shows the high speed TIA connections to the switch

matrix and external pins. Note the extra load and gain resistors,

RLOAD_DE0 and RTIA_DE0, respectively, available on the DE0 pin.

Figure 29. High Speed TIA Switches

External RTIA Selection

The high speed TIA has the option of selecting an external gain

resistor instead of the internal RTIA gain options. To perform this

selection, connect one end of the resistor to the DE0 pin and

connect the other end to AIN0, AIN1, AIN2, or AIN3/

BUF_VREF1V8. The DE0 pin must be connected to the output

of the high speed TIA.

To use the DE0 pin for the external RTIA value, set the following

•

DE0RESCON = 0x97.

•

HSRTIACON, Bits[3:0] = 0xF.

AIN0, AIN1, AIN2, or AIN3/BUF_VREF1V8 (whichever pin

the resistor is connected to) must be connected to the inverting

input of the high speed TIA (see the Programmable Switch

Matrix section). When DE0RESCON = 0x97, the RLOAD_DE0 and

RTIA_DE0 resistors are short circuit, which means that the external

RTIA is connected directly to the output of the high speed TIA.

Figure 30. Connecting External RTIA to the High Speed TIA

Tx/TR1

SWITCHES

–

RTIA

HIGH SPEED

TRANSIMPEDANCE

AMPLIFIER

CTIA

TR1

TIA INPUT

HSTIA

OUTPUT

RTIA_DE0

RLOAD_DE0

DE0RESCON[7:0]

HSRTIACON[12:5]

HSRTIACON[3:0]

HSRTIACON[4]

SWITCH AND RLOAD

CONTROLLED BY

DE0RESCON[7:0]

T10

DE0

SE0

AIN0

AIN1

AIN2

AIN3

RLOAD_SE0

SW6

–

HIGH SPEED

TRANSIMPEDANCE

AMPLIFIER

HSTIA

OUTPUT

DE0RESCON

T10

RTIA

CTIA

HSRTIACON[3:0]

HSRTIACON[12:5]

DE0RESCON[7:0]

RLOAD_DE0

DE0RESCON[7:0]

DE0

AIN1

EXTERNAL

RTIA

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