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User Manual: Family of 8-bit Digitizers Page 29 of 66
3.2. Channel Input
3.2.1. Coupling
Both AC and DC coupling modes are available. The AC mode couples signals capacitively thus removing the input
signal’s DC component and filtering out any signal component below 16 Hz for the DC2x1A digitizers, 32 Hz for the
DC271-Family and 10 Hz for all other models. DC mode allows all signal components to be passed through to the
digitizer.
3.2.2. Impedance
The input channels of the DP and DC Series digitizers offer termination into 50 Ω and, sometimes 1 MΩ. The 50 Ω
coupling mode offers high quality termination with better than ± 1% precision. It is ideally suited to use with 50 Ω
transmission lines (coax), high bandwidth low impedance (typically 500 Ω) probes or active probes. The 1 MΩ
coupling mode provides a high impedance (low load) capability that is suited for use with most standard high
impedance probes. The high impedance mode also features low (typically ~10 pF, see table) capacitance that helps to
minimize loading effects that can occur when probing high frequency circuits.
3.2.3. Input Protection
The input amplifiers are fully protected against over-voltage signals. Input signals up to ±300 V for the DC271A,
DC271AR, DC241A, and DC211A and ±100 V otherwise (DC + peak AC < 10 KHz) at 1 MΩ and up to ±5 V DC at
50 Ω, can be input without damaging the front-end electronics.
The front-end also provides an overload protection that will automatically switch the coupling from 50 Ω to 1 MΩ if
the signal is greater than ±5 V DC. This is implemented for most modules (not the DC2x1A) that have the 1MΩ
impedance possibility.
3.2.4. Mezzanine Front-end
The front-end electronics are all mounted on a removable mezzanine card. In the event of accidental damage, or as
components fatigue over time (e.g. relays in high duty cycle automated testing applications), the mezzanine card
allows for fast and efficient replacement.
3.2.5. Bandwidth and Rise Time
The bandwidth specification indicates the frequency at which an input signal will be attenuated by 3 dB
(approximately 30% loss of amplitude). The bandwidth also affects the minimum rise and fall times that can be
passed through the front-end electronics. A pulse with a very sharp edge will be observed to have a minimum rise
time (τ
min
) determined by the front-end electronics. In general a pulse with a given 10-90% rise time (τ
10-90real
) will be
observed with a slower value given by:
τ
10-90
2
= τ
10-90real
2
+ τ
min
2
where τ
min
(ns) ≈ 0.35 (GHz-ns) / BW (GHz)
The following table indicates the bandwidth and rise time performance of the DP and DC Series digitizers. For many
digitizers, the bandwidth can be limited with hardware bandwidth limiters as shown in the tables below. For many of
the older models the bandwidth at 50 mV FS, BW
50
, is not as high as that of the other sensitivities.