TSA1204(2006) View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

TSA1204
(Rev.:2006)

Dual channel 12-bit 20Msps 120mW A/D converter

STMicroelectronics 

TSA1204

Application information

8.3

Driving the differential analog inputs

The TSA1204 is designed to deliver optimum performance when driven on differential

inputs. An RF transformer is an efficient way of achieving this high performance.

Figure 25 describes the schematics. The input signal is fed to the primary of the transformer,

while the secondary drives both ADC inputs. The common mode voltage of the ADC (INCM)

is connected to the center-tap of the secondary of the transformer in order to bias the input

signal around this common voltage, internally set to 0.46 V. It determines the DC component

of the analog signal. Being a high impedance input, it acts as an I/O and can be externally

driven to adjust this DC component. The INCM is decoupled to maintain a low noise level on

this node. Our evaluation board is mounted with a 1:1 ADT1-1WT transformer from

Minicircuits. You might also use a higher impedance ratio (1:2 or 1:4) to reduce the driving

requirement on the analog signal source.

Each analog input can drive a 1.4 Vpp amplitude input signal, so the resulting differential

amplitude is 2.8 Vpp.

Figure 25. Differential input configuration with transformer

Analog source

ADT1-1

1:1

50Ω

33pF

VIN

TSA1204

channels

VINB I or Q

INCM

330pF 10nF 470nF

Figure 26. AC-coupled differential input

50Ω

common

mode

10nF

33pF

50Ω 10nF

100kΩ

100kΩ

VIN

INCM TSA1204

VINB

Figure 26 represents the biasing of a differential input signal in AC-coupled differential input

configuration. Both inputs VIN and VINB are centered around the common mode voltage, that

can be let internal or fixed externally.

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