Datasheet

300: TWISTED PAIR

4 V

1:2 (TURNS)

37.5:

8 V

500

250

= 2

= 300:

1V LOW IMPEDANCE

VOLTAGE REFERENCE

ADC12LO66

39 pF

7 - 8 pF

REF

LMH6551

SNOSAK7C –FEBRUARY 2005–REVISED MARCH 2013

www.ti.com

Figure 33. Driving an ADC

The amplifier and ADC should be located as closely together as possible. Both devices require that the filter

components be in close proximity to them. The amplifier needs to have minimal parasitic loading on the output

traces and the ADC is sensitive to high frequency noise that may couple in on its input lines. Some high

performance ADCs have an input stage that has a bandwidth of several times its sample rate. The sampling

process results in all input signals presented to the input stage mixing down into the Nyquist range (DC to Fs/2).

See AN-236 for more details on the subsampling process and the requirements this imposes on the filtering

necessary in your system.

USING TRANSFORMERS

Transformers are useful for impedance transformation as well as for single to differential, and differential to single

ended conversion. A transformer can be used to step up the output voltage of the amplifier to drive very high

impedance loads as shown in Figure 34. Figure 36 shows the opposite case where the output voltage is stepped

down to drive a low impedance load.

Transformers have limitations that must be considered before choosing to use one. Compared to a differential

amplifier, the most serious limitations of a transformer are the inability to pass DC and balance error (which

causes distortion and gain errors). For most applications the LMH6551 will have adequate output swing and drive

current and a transformer will not be desirable. Transformers are used primarily to interface differential circuits to

50Ω single ended test equipment to simplify diagnostic testing.

Figure 34. Transformer Out High Impedance Load

Product Folder Links: LMH6551